Set _NEW_BUFFERS in glRead/DrawBuffer().

Set _NEW_BUFFERS in glRead/DrawBuffer().

Previously, we set _NEW_PIXEL and _NEW_COLOR in these functions, respectively.
That correponds to the GL attribute groups, but doesn't make much sense
otherwise.  This could improve validation efficiency in a few places too.
It looks like all the drivers are already checking for _NEW_BUFFERS in the
right places (since that's the bit for FBO state) so we can trim out
_NEW_PIXEL and _NEW_COLOR at any time.