meta: fix glDrawPixels fallback test, stencil drawing

meta: fix glDrawPixels fallback test, stencil drawing

Remove the check for pixel transfer ops.  If any RGB/depth scale/bias
is in effect, it'll be applied in the glTexImage step.

If drawing stencil pixels we need to disable pixel transfer so that
alpha scale/bias are not applied to the stencil data.

These issues were spotted by Roland.

Fixes Blender performance issues reported in
http://bugs.freedesktop.org/show_bug.cgi?id=47375

NOTE: This is a candidate for the 8.0 branch.

Tested-by: Barto <mister.freeman@laposte.net>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>