mesa: convert some #defines to enums

mesa: convert some #defines to enums

This makes debugging with gdb a bit easier.
Ex:
(gdb) p ctx->DrawBuffer.Attachment[BUFFER_STENCIL]

Note however that gdb only seems to recognize enum types that are actually
used to declare a variable somewhere.  For example, gl_buffer_index isn't
used to declare any vars so it's invisible to gdb.  Work around this by
adding a dummy function in context.c that declares some vars with these
new types.