mesa/main: add support for EXT_texture_storage
mesa/main: add support for EXT_texture_storage
It's sometimes really, really useful if GL_BGRA8 can be used as a sized internal format, and the combination of EXT_texture_storage and EXT_texture_format_BGRA8888 allows this (only when using texture-storage, which is good enough in some cases). Until now, we've only implemented ARB_texture_storage, and not the EXT version.
So let's implement the EXT version as well, so we get the benefit of the interaction here. This pulls in a lot of other similar interactions as well, which also seems useful.