intel: Partially fix fallback detection for glCopyTexSubImage.

intel: Partially fix fallback detection for glCopyTexSubImage.

Really, we should be checking that the MesaTexFormat for the read buffer
and the texture match, but the previous code wasn't even doing that, so
matching the cpp should be an improvement (and potentially fix some hangs!).
The previous code also rejected GL_RGB even though blitting the alpha
channel to the ignored channel of an x8r8g8b8 texture should be fine, which
tripped up compiz's blur plugin.