Make use of emscripten_supports_offscreencanvas. NFC (#21204)

Author: sbc100

src/library_html5_webgl.js

@@ -45,6 +45,7 @@ var LibraryHtml5WebGL = {
   emscripten_webgl_do_create_context__deps: [
 #if OFFSCREENCANVAS_SUPPORT
   'malloc',
+  'emscripten_supports_offscreencanvas',
 #endif
 #if PTHREADS && OFFSCREEN_FRAMEBUFFER
   'emscripten_webgl_create_context_proxied',
@@ -95,7 +96,7 @@ var LibraryHtml5WebGL = {
         dbg('Performance warning: forcing renderViaOffscreenBackBuffer=true and preserveDrawingBuffer=true since proxying WebGL rendering.');
 #endif
         // We will be proxying - if OffscreenCanvas is supported, we can proxy a bit more efficiently by avoiding having to create an Offscreen FBO.
-        if (typeof OffscreenCanvas == 'undefined') {
+        if (!_emscripten_supports_offscreencanvas()) {
           {{{ makeSetValue('attributes', C_STRUCTS.EmscriptenWebGLContextAttributes.renderViaOffscreenBackBuffer, '1', 'i32') }}};
           {{{ makeSetValue('attributes', C_STRUCTS.EmscriptenWebGLContextAttributes.preserveDrawingBuffer, '1', 'i32') }}};
         }
@@ -115,12 +116,12 @@ var LibraryHtml5WebGL = {
     if (canvas.offscreenCanvas) canvas = canvas.offscreenCanvas;
 
 #if GL_DEBUG
-    if (typeof OffscreenCanvas != 'undefined' && canvas instanceof OffscreenCanvas) dbg(`emscripten_webgl_create_context: Creating an OffscreenCanvas-based WebGL context on target "${targetStr}"`);
+    if (_emscripten_supports_offscreencanvas() && canvas instanceof OffscreenCanvas) dbg(`emscripten_webgl_create_context: Creating an OffscreenCanvas-based WebGL context on target "${targetStr}"`);
     else if (typeof HTMLCanvasElement != 'undefined' && canvas instanceof HTMLCanvasElement) dbg(`emscripten_webgl_create_context: Creating an HTMLCanvasElement-based WebGL context on target "${targetStr}"`);
 #endif
 
     if (contextAttributes.explicitSwapControl) {
-      var supportsOffscreenCanvas = canvas.transferControlToOffscreen || (typeof OffscreenCanvas != 'undefined' && canvas instanceof OffscreenCanvas);
+      var supportsOffscreenCanvas = canvas.transferControlToOffscreen || (_emscripten_supports_offscreencanvas() && canvas instanceof OffscreenCanvas);
 
       if (!supportsOffscreenCanvas) {
 #if OFFSCREEN_FRAMEBUFFER

test/canvas_animate_resize.cpp

@@ -126,14 +126,14 @@ int main()
   printf("Created context with handle %u\n", (unsigned int)ctx);
   if (!ctx)
   {
-    EM_ASM({
-      if (typeof OffscreenCanvas === 'undefined') {
+    if (!emscripten_supports_offscreencanvas()) {
+      EM_ASM({
         xhr = new XMLHttpRequest();
         xhr.open('GET', 'http://localhost:8888/report_result?skipped:%20OffscreenCanvas%20is%20not%20supported!');
         xhr.send();
         setTimeout(function() { window.close() }, 2000);
-      }
-    });
+      });
+    }
     return 0;
   }
   emscripten_webgl_make_context_current(ctx);