[OffscreenCanvas] Extract the canvas from the offscreenCanvases entry (#22958)

Fixes a type error in #22943 when creating a context with explicit swap
control.

Author: JoeOsborn

src/library_html5_webgl.js

@@ -171,7 +171,7 @@ var LibraryHtml5WebGL = {
 #endif
           return 0;
         }
-        canvas = GL.offscreenCanvases[canvas.id];
+        canvas = GL.offscreenCanvases[canvas.id].canvas;
       }
     }
 #else // !OFFSCREENCANVAS_SUPPORT

test/browser/webgl_create_context_swapcontrol.c

@@ -0,0 +1,27 @@
+// Copyright 2024 The Emscripten Authors.  All rights reserved.
+// Emscripten is available under two separate licenses, the MIT license and the
+// University of Illinois/NCSA Open Source License.  Both these licenses can be
+// found in the LICENSE file.
+
+#include <GLES2/gl2.h>
+#include <GLES2/gl2ext.h>
+#include <assert.h>
+
+#include <emscripten/emscripten.h>
+#include <emscripten/html5.h>
+
+int main() {
+  EmscriptenWebGLContextAttributes attrs;
+  emscripten_webgl_init_context_attributes(&attrs);
+  attrs.explicitSwapControl = true;
+#ifdef USE_OFFSCREEN_FRAMEBUFFER
+  attrs.renderViaOffscreenBackBuffer = true;
+#endif
+  // Test that creating a context succeeds
+  EMSCRIPTEN_WEBGL_CONTEXT_HANDLE context = emscripten_webgl_create_context("#canvas", &attrs);
+  assert(context > 0); // Must have received a valid context.
+  EMSCRIPTEN_RESULT res = emscripten_webgl_make_context_current(context);
+  assert(res == EMSCRIPTEN_RESULT_SUCCESS);
+  assert(emscripten_webgl_get_current_context() == context);
+  return 0;
+}

test/test_browser.py

@@ -2612,6 +2612,15 @@ def test_html5_webgl_create_context(self, args):
   def test_html5_webgl_create_context2(self):
     self.btest_exit('webgl_create_context2.cpp')
 
+  @requires_graphics_hardware
+  # Verify bug https://github.com/emscripten-core/emscripten/issues/22943: creating a WebGL context with explicit swap control and offscreenCanvas
+  @parameterized({
+    'offscreencanvas': (['-sOFFSCREENCANVAS_SUPPORT'],),
+    'offscreenframebuffer': (['-sOFFSCREEN_FRAMEBUFFER', '-DUSE_OFFSCREEN_FRAMEBUFFER'],),
+  })
+  def test_html5_webgl_create_context_swapcontrol(self, args):
+    self.btest_exit('browser/webgl_create_context_swapcontrol.c', args=args)
+
   @requires_graphics_hardware
   # Verify bug https://github.com/emscripten-core/emscripten/issues/4556: creating a WebGL context to Module.canvas without an ID explicitly assigned to it.
   # (this only makes sense in the old deprecated -sDISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=0 mode)