i965: Rename some vague format members of brw_context

i965: Rename some vague format members of brw_context

I'm swimming in a vortex of formats. Mesa formats, isl formats, DRI
formats, GL formats, etc.

It's easy to misinterpret the following brw_context members unless
you've recently read their definition.  In upcoming patches, I change
them from embedded arrays to simple pointers; after that, even their
definition doesn't help, because the MESA_FORMAT_COUNT hint will no
longer be present.

Rename them to prevent further confusion. While we're renaming, choose
shorter names too.

    -format_supported_as_render_target
    +mesa_format_supports_render

    -render_target_format
    +mesa_to_isl_render_format

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>