mesa: Ignore blits to/from missing buffers

mesa: Ignore blits to/from missing buffers

The EXT_framebuffer_object spec (and later specs) say:

     "If a buffer is specified in <mask> and does not exist in both
     the read and draw framebuffers, the corresponding bit is silently
     ignored."

Check for color, depth, and stencil that the source and destination
FBOs have the specified buffers.  If the buffer is missing, remove the
bit from the blit request mask and continue.

Fixes the crash in piglit test 'fbo-missing-attachment-blit from', and
fixes 'fbo-missing-attachment-blit es2 from'.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=37739
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
NOTE: This is a candidate for the stable branches.