bug in glamor on ShmPutImage for XY images

[Jamey Sharp]

I haven't understood enough of the implementation to see how to fix
this, but I can reliably crash Xephyr this way:

hw/kdrive/ephyr/Xephyr :1 -screen 1024x768 -glamor
DISPLAY=:1 x11perf -shmputxy10

Without the -glamor option to Xephyr, the x11perf command runs
successfully. I'm testing on current git master.

As far as I can tell, fbPutXYImage is getting called with a pDrawable
for which the pixmap's devPrivate.ptr is NULL. It's setting FbBits
*dst to that NULL and passing it down to fbBltOne, which then
segfaults.

I guess ShmPutImage works fine for ZPixmaps and core PutImage works
for XYPixmaps; I haven't seen anything fail except when the two are
put together.

I kind of wanted to try the performance comparison exercise that
keithp described in http://keithp.com/blogs/glamor-hacking/ but this
got in the way.

Jamey