Fix Getting WebGPU Context in pthread (#21467)

Failed to run compiled WGPU code clips:
 wgpu::SurfaceDescriptor surfDesc{};
 surfDesc.nextInChain = &canvasDesc;
 wgpu::Surface surface = instance.CreateSurface(&surfDesc);

The reason is that it cannot call getContext from correct canvas
object.

This PR fixed the problem and add correct build flags for
webgpu_basic_rendering test in pthread.

Author: shaoboyan

src/library_webgpu.js

@@ -2431,6 +2431,9 @@ var LibraryWebGPU = {
     var selectorPtr = {{{ makeGetValue('descriptorFromCanvasHTMLSelector', C_STRUCTS.WGPUSurfaceDescriptorFromCanvasHTMLSelector.selector, '*') }}};
     {{{ gpu.makeCheck('selectorPtr') }}}
     var canvas = findCanvasEventTarget(selectorPtr);
+#if OFFSCREENCANVAS_SUPPORT
+    if (canvas.offscreenCanvas) canvas = canvas.offscreenCanvas;
+#endif
     var context = canvas.getContext('webgpu');
 #if ASSERTIONS
     assert(context);

test/test_browser.py

@@ -4689,17 +4689,19 @@ def test_webgl_simple_extensions(self, simple_enable_extensions, webgl_version):
   def test_webgpu_basic_rendering(self, args):
     self.btest_exit('webgpu_basic_rendering.cpp', args=['-sUSE_WEBGPU'] + args)
 
+  # TODO(#19645): Extend this test to proxied WebGPU when it's re-enabled.
   @requires_graphics_hardware
   @requires_threads
   def test_webgpu_basic_rendering_pthreads(self):
-    self.btest_exit('webgpu_basic_rendering.cpp', args=['-sUSE_WEBGPU', '-pthread', '-sPROXY_TO_PTHREAD'])
+    self.btest_exit('webgpu_basic_rendering.cpp', args=['-sUSE_WEBGPU', '-pthread', '-sOFFSCREENCANVAS_SUPPORT'])
 
   def test_webgpu_get_device(self):
     self.btest_exit('webgpu_get_device.cpp', args=['-sUSE_WEBGPU', '-sASSERTIONS', '--closure=1'])
 
+  # TODO(#19645): Extend this test to proxied WebGPU when it's re-enabled.
   @requires_threads
   def test_webgpu_get_device_pthreads(self):
-    self.btest_exit('webgpu_get_device.cpp', args=['-sUSE_WEBGPU', '-pthread', '-sPROXY_TO_PTHREAD'])
+    self.btest_exit('webgpu_get_device.cpp', args=['-sUSE_WEBGPU', '-pthread'])
 
   # Tests the feature that shell html page can preallocate the typed array and place it
   # to Module.buffer before loading the script page.