WIP: Add a new image format enum for use in the compiler

Currently, we're just using the GL format enum. However, we would like to be able to extend it to BGRA which doesn't work because GL doesn't have those formats. This adds a new enum to shader_enums.h for these formats. The enum takes on exactly the same values as the GL enums so it can be used interchangeably as long as the value being used is one of the GL values. We could, in theory, use MESA_FORMAT_* however that's currently a GL-only thing and this needs to transcend GL.

At one point, I send this to the mailing list and @anholt NAK'd it. Maybe he's changed his mind?