hal/gles: work around the clear shader bug

CHANGELOG.md

@@ -2,7 +2,11 @@
 
 ## wgpu-hal-0.11.1 (2021-10-09)
   - Vulkan: fix NV optimus detection on Linux
-  - WebGL: fix querying storage-related limits
+  - GL:
+    - fix indirect dispatch buffers
+  - WebGL:
+    - fix querying storage-related limits
+    - work around a browser bug in the clear shader
 
 ## wgpu-0.11 (2021-10-07)
   - Infrastructure:

wgpu-hal/src/gles/shader_clear.frag

@@ -1,7 +1,9 @@
 #version 300 es
 precision lowp float;
 uniform vec4 color;
+//Hack: Some WebGL implementations don't find "color" otherwise.
+uniform vec4 color_workaround;
 out vec4 frag;
 void main() {
-  frag = color;
-}
+  frag = color + color_workaround;
+}