[Mesa-dev] Fixing webgl crashes

[Eric Anholt]

This is a series to fix our Firefox WebGL crashes.  Their debug code,
which they do nightly testing on, uses a debug mode which calls
glFinish() after every operation.  That revealed failures in our
glFinish() implementation.

I'm not really pleased with the state handling for
_ColorDrawBuffers[].  Non-refcounted pointers to refcounted objects
fill me with rage (after filling my bugzilla with crashes).  This is
sort of the minimal patch, while I think a more invasive patch would
be to remove _ColorDrawBuffers[] and just give people an accessor
function using _ColorDrawIndexes[] which are the indexes into
Attachments[] that are updated immediately upon glDrawBuffers().