Workaround drisw bug where clipped redraws don't work

This detects when we are running on any of Mesa's software rasterizer
backends and disables use of glBlitFramebuffer and glXCopySubBuffer.
Both of these currently result in full-screen copies so there's little
point in using these to optimize how much of the screen we present.

To help ensure we re-evaluate this workaround periodically we have added
a comment marker of "ONGOING BUG" above the workaround and added a note
to our RELEASING document that says we should grep for this marker and
write a NEWS section about ongoing bug workarounds.

https://bugzilla.gnome.org/show_bug.cgi?id=674208



Reviewed-by: Neil Roberts <neil@linux.intel.com>