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.cpp

@@ -30,6 +30,8 @@ static GLenum to_gl_enum(BlendFactor factor)
         return GL_SRC_ALPHA;
     case BlendFactor::One:
         return GL_ONE;
+    case BlendFactor::Zero:
+        return GL_ZERO;
     case BlendFactor::OneMinusSrcAlpha:
         return GL_ONE_MINUS_SRC_ALPHA;
     }