Keith Packard authored Mar 15, 2014 and Emma Anholt committed Jun 15, 2014

These offer a simpler and more efficient means for temporarily
transitioning to CPU-accessible memory for fallback implementations.

v2: Do not attempt fallbacks with GLAMOR_DRM_ONLY pixmaps

    glamor cannot transfer pixels for GLAMOR_DRM_ONLY pixmaps using
    glReadPixels and glTexSubImage2D, and so there's no way to perform
    fallback operations with these pixmaps.

v3: Clear ->pbo field when deleting the PBO.  Otherwise, we'd reuse
    the old name next time we fall back on the pixmap, which would
    potentially conflict with some other pixmap that genned a new
    name, or just do a lazy allocation of the name (compat GL context,
    like we currently use) or error out (core GL context, like we hope
    to use some day).  Also, style fixes.  Changes by anholt, acked by
    keithp.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Eric Anholt <eric@anholt.net>