mesa/main: allow GL_BGRA for FBOs

The EXT_texture_format_BGRA8888 spec clearly defines GL_BGRA as a color-renderable format, so we need to support it here as well.

This has been broken since the day support for the extension was added. Oh well, let's fix it up!

Fixes: 1d595c7c ("gles2: Add GL_EXT_texture_format_BGRA8888 support")