LibWeb: Implement border radius corner clipping in GPU painter

It is implemented in the way identical to how it works in CPU painter:
1. SampleUnderCorners command saves pixels within corners into a
   texture.
2. BlitCornerClipping command uses the texture prepared earlier to
   restore pixels within corners.

Userland/Libraries/LibAccelGfx/GL.h

@@ -61,6 +61,7 @@ struct Framebuffer {
 void set_viewport(Gfx::IntRect);
 
 enum class BlendFactor {
+    Zero,
     One,
     OneMinusSrcAlpha,
     SrcAlpha,