emscripten: Add a hint for specifying the canvas selector

Now that we're not going through EGL, this is easy

include/SDL_hints.h

@@ -366,6 +366,15 @@ extern "C" {
  */
 #define SDL_HINT_EMSCRIPTEN_ASYNCIFY   "SDL_EMSCRIPTEN_ASYNCIFY"
 
+/**
+ *  \brief Specify the CSS selector used for the "default" window/canvas
+ *
+ * This hint only applies to the emscripten platform
+ *
+ * The default value is "#canvas"
+ */
+#define SDL_HINT_EMSCRIPTEN_CANVAS_SELECTOR "SDL_EMSCRIPTEN_CANVAS_SELECTOR"
+
 /**
  *  \brief override the binding element for keyboard inputs for Emscripten builds
  *

src/video/emscripten/SDL_emscriptenvideo.c

@@ -215,6 +215,7 @@ Emscripten_CreateWindow(_THIS, SDL_Window * window)
     SDL_WindowData *wdata;
     double scaled_w, scaled_h;
     double css_w, css_h;
+    const char *selector;
 
     /* Allocate window internal data */
     wdata = (SDL_WindowData *) SDL_calloc(1, sizeof(SDL_WindowData));
@@ -222,7 +223,12 @@ Emscripten_CreateWindow(_THIS, SDL_Window * window)
         return SDL_OutOfMemory();
     }
 
-    wdata->canvas_id = SDL_strdup("#canvas");
+    selector = SDL_GetHint(SDL_HINT_EMSCRIPTEN_CANVAS_SELECTOR);
+    if (!selector) {
+        selector = "#canvas";
+    }
+
+    wdata->canvas_id = SDL_strdup(selector);
 
     if (window->flags & SDL_WINDOW_ALLOW_HIGHDPI) {
         wdata->pixel_ratio = emscripten_get_device_pixel_ratio();