Fix context creation of transferred offscreen canvas (#23518)

After a canvas has been transferred offscreen, it can't be used again
to create a webgl context because the result of findCanvasEventTarget
is a {canvas:...} map instead of an actual canvas object. This patch grabs
the underlying offscreencanvas if it exists.

Author: JoeOsborn

src/lib/libhtml5_webgl.js

@@ -109,7 +109,10 @@ var LibraryHtml5WebGL = {
 #endif
 
     var canvas = findCanvasEventTarget(target);
-
+#if OFFSCREENCANVAS_SUPPORT
+    // If our canvas from findCanvasEventTarget is actually an offscreen canvas record, we should extract the inner canvas.
+    if (canvas?.canvas) { canvas = canvas.canvas; }
+#endif
 #if GL_DEBUG
     var targetStr = UTF8ToString(target);
 #endif

test/browser/webgl_create_context_swapcontrol.c

@@ -23,5 +23,13 @@ int main() {
   EMSCRIPTEN_RESULT res = emscripten_webgl_make_context_current(context);
   assert(res == EMSCRIPTEN_RESULT_SUCCESS);
   assert(emscripten_webgl_get_current_context() == context);
+  emscripten_webgl_destroy_context(context);
+  // Test that creating a context for a canvas that's already been used once succeeds
+  context = emscripten_webgl_create_context("#canvas", &attrs);
+  assert(context > 0); // Must have received a valid context.
+  res = emscripten_webgl_make_context_current(context);
+  assert(res == EMSCRIPTEN_RESULT_SUCCESS);
+  assert(emscripten_webgl_get_current_context() == context);
+  emscripten_webgl_destroy_context(context);
   return 0;
 }