x11: Minor cleanup in new mouse cursor selection code.

- Move legacy name choice to a separate function, so we can `return` a
  string in one line instead of assign a variable and `break` for each item.
- Have the case statement cover SDL_NUM_SYSTEM_CURSORS, and not `default`, so
  compiler will (maybe) warn us if an enum value is added but not included
  here.
- Only choose a legacy name if necessary.

(cherry picked from commit df00a7dd4c3deb03839e799187a3c75fc4e8854b)

src/video/x11/SDL_x11mouse.c

@@ -213,97 +213,61 @@ static SDL_Cursor *X11_CreateCursor(SDL_Surface *surface, int hot_x, int hot_y)
     return cursor;
 }
 
-static SDL_Cursor *X11_CreateSystemCursor(SDL_SystemCursor id)
+static unsigned int GetLegacySystemCursorShape(SDL_SystemCursor id)
 {
-    SDL_Cursor *cursor = NULL;
-    unsigned int shape = 0;
-    const char *xcursorname = SDL_GetCSSCursorName(id, NULL);
-
     switch (id) {
-    default:
-        SDL_assert(0);
-        return NULL;
-    /* X Font Cursors reference: */
-    /*   http://tronche.com/gui/x/xlib/appendix/b/ */
-    case SDL_SYSTEM_CURSOR_ARROW:
-        shape = XC_left_ptr;
-        break;
-    case SDL_SYSTEM_CURSOR_IBEAM:
-        shape = XC_xterm;
-        break;
-    case SDL_SYSTEM_CURSOR_WAIT:
-        shape = XC_watch;
-        break;
-    case SDL_SYSTEM_CURSOR_CROSSHAIR:
-        shape = XC_tcross;
-        break;
-    case SDL_SYSTEM_CURSOR_WAITARROW:
-        shape = XC_watch;
-        break;
-    case SDL_SYSTEM_CURSOR_SIZENWSE:
-        shape = XC_top_left_corner;
-        break;
-    case SDL_SYSTEM_CURSOR_SIZENESW:
-        shape = XC_top_right_corner;
-        break;
-    case SDL_SYSTEM_CURSOR_SIZEWE:
-        shape = XC_sb_h_double_arrow;
-        break;
-    case SDL_SYSTEM_CURSOR_SIZENS:
-        shape = XC_sb_v_double_arrow;
-        break;
-    case SDL_SYSTEM_CURSOR_SIZEALL:
-        shape = XC_fleur;
-        break;
-    case SDL_SYSTEM_CURSOR_NO:
-        shape = XC_pirate;
-        break;
-    case SDL_SYSTEM_CURSOR_HAND:
-        shape = XC_hand2;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_TOPLEFT:
-        shape = XC_top_left_corner;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_TOP:
-        shape = XC_top_side;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_TOPRIGHT:
-        shape = XC_top_right_corner;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_RIGHT:
-        shape = XC_right_side;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_BOTTOMRIGHT:
-        shape = XC_bottom_right_corner;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_BOTTOM:
-        shape = XC_bottom_side;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_BOTTOMLEFT:
-        shape = XC_bottom_left_corner;
-        break;
-    case SDL_SYSTEM_CURSOR_WINDOW_LEFT:
-        shape = XC_left_side;
-        break;
+        /* X Font Cursors reference: */
+        /*   http://tronche.com/gui/x/xlib/appendix/b/ */
+        case SDL_SYSTEM_CURSOR_ARROW: return XC_left_ptr;
+        case SDL_SYSTEM_CURSOR_IBEAM: return XC_xterm;
+        case SDL_SYSTEM_CURSOR_WAIT: return XC_watch;
+        case SDL_SYSTEM_CURSOR_CROSSHAIR: return XC_tcross;
+        case SDL_SYSTEM_CURSOR_WAITARROW: return XC_watch;
+        case SDL_SYSTEM_CURSOR_SIZENWSE: return XC_top_left_corner;
+        case SDL_SYSTEM_CURSOR_SIZENESW: return XC_top_right_corner;
+        case SDL_SYSTEM_CURSOR_SIZEWE: return XC_sb_h_double_arrow;
+        case SDL_SYSTEM_CURSOR_SIZENS: return XC_sb_v_double_arrow;
+        case SDL_SYSTEM_CURSOR_SIZEALL: return XC_fleur;
+        case SDL_SYSTEM_CURSOR_NO: return XC_pirate;
+        case SDL_SYSTEM_CURSOR_HAND: return XC_hand2;
+        case SDL_SYSTEM_CURSOR_WINDOW_TOPLEFT: return XC_top_left_corner;
+        case SDL_SYSTEM_CURSOR_WINDOW_TOP: return XC_top_side;
+        case SDL_SYSTEM_CURSOR_WINDOW_TOPRIGHT: return XC_top_right_corner;
+        case SDL_SYSTEM_CURSOR_WINDOW_RIGHT: return XC_right_side;
+        case SDL_SYSTEM_CURSOR_WINDOW_BOTTOMRIGHT: return XC_bottom_right_corner;
+        case SDL_SYSTEM_CURSOR_WINDOW_BOTTOM: return XC_bottom_side;
+        case SDL_SYSTEM_CURSOR_WINDOW_BOTTOMLEFT: return XC_bottom_left_corner;
+        case SDL_SYSTEM_CURSOR_WINDOW_LEFT: return XC_left_side;
+        case SDL_NUM_SYSTEM_CURSORS: break;  /* so the compiler might notice if an enum value is missing here. */
     }
 
-    cursor = SDL_calloc(1, sizeof(*cursor));
-    if (cursor) {
-        Display *dpy = GetDisplay();
-        Cursor x11_cursor = None;
+    SDL_assert(0);
+    return 0;
+}
+
+static SDL_Cursor *X11_CreateSystemCursor(SDL_SystemCursor id)
+{
+    SDL_Cursor *cursor = NULL;
+    Display *dpy = GetDisplay();
+    Cursor x11_cursor = None;
 
 #ifdef SDL_VIDEO_DRIVER_X11_XCURSOR
-        SDL_assert(xcursorname != NULL);
-        if (SDL_X11_HAVE_XCURSOR) {
-            x11_cursor = X11_XcursorLibraryLoadCursor(dpy, xcursorname);
-        }
+    if (SDL_X11_HAVE_XCURSOR) {
+        x11_cursor = X11_XcursorLibraryLoadCursor(dpy, SDL_GetCSSCursorName(id, NULL));
+    }
 #endif
 
-        if (x11_cursor == None) {
-            x11_cursor = X11_XCreateFontCursor(dpy, shape);
-        }
+    if (x11_cursor == None) {
+        x11_cursor = X11_XCreateFontCursor(dpy, GetLegacySystemCursorShape(id));
+    }
 
-        cursor->driverdata = (void *)(uintptr_t)x11_cursor;
+    if (x11_cursor != None) {
+        cursor = SDL_calloc(1, sizeof(*cursor));
+        if (!cursor) {
+            SDL_OutOfMemory();
+        } else {
+            cursor->driverdata = (void *)(uintptr_t)x11_cursor;
+        }
     }
 
     return cursor;