The extension was already implemented for GLES3, but simply required
a few cases to be added for support in GLES2.

Note that this extension is required to process filters using masks
with GLES2 in skia.

Change-Id: I196901ca19a4fd40a5a1ebd87040519b0de6eb5f
Reviewed-on: https://swiftshader-review.googlesource.com/9949Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>