blit_slow: don't read destination pixel when you're going to discard it anyways

(cherry picked from commit 4cd0c13823551cce279c9ff3d284c41c28deb3d8)

src/video/SDL_blit_slow.c

@@ -106,15 +106,20 @@ void SDL_Blit_Slow(SDL_BlitInfo *info)
                     continue;
                 }
             }
-            if (FORMAT_HAS_ALPHA(dstfmt_val)) {
-                DISEMBLE_RGBA(dst, dstbpp, dst_fmt, dstpixel, dstR, dstG, dstB, dstA);
-            } else if (FORMAT_HAS_NO_ALPHA(dstfmt_val)) {
-                DISEMBLE_RGB(dst, dstbpp, dst_fmt, dstpixel, dstR, dstG, dstB);
-                dstA = 0xFF;
+            if ((flags & (SDL_COPY_BLEND | SDL_COPY_ADD | SDL_COPY_MOD | SDL_COPY_MUL))) {
+                if (FORMAT_HAS_ALPHA(dstfmt_val)) {
+                    DISEMBLE_RGBA(dst, dstbpp, dst_fmt, dstpixel, dstR, dstG, dstB, dstA);
+                } else if (FORMAT_HAS_NO_ALPHA(dstfmt_val)) {
+                    DISEMBLE_RGB(dst, dstbpp, dst_fmt, dstpixel, dstR, dstG, dstB);
+                    dstA = 0xFF;
+                } else {
+                    /* SDL_PIXELFORMAT_ARGB2101010 */
+                    dstpixel = *((Uint32 *) (dst));
+                    RGBA_FROM_ARGB2101010(dstpixel, dstR, dstG, dstB, dstA);
+                }
             } else {
-                /* SDL_PIXELFORMAT_ARGB2101010 */
-                dstpixel = *((Uint32 *)(dst));
-                RGBA_FROM_ARGB2101010(dstpixel, dstR, dstG, dstB, dstA);
+                /* don't care */
+                dstR = dstG = dstB = dstA = 0;
             }
 
             if (flags & SDL_COPY_MODULATE_COLOR) {