These were already available for GLES3 and exposed as texturable
formats for GLES2 when the GL_EXT_texture_rg extension was exposed,
but it should also have enabled these formats as renderable formats.

Bug swiftshader:60

Change-Id: I598d7645b04737dae19dc88b4229fa3d7d231714
Reviewed-on: https://swiftshader-review.googlesource.com/9990Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>