I think that a major cost in the current implementation is doing a
CreateDIBSection()/DestroyObject() on every refresh.  So provide our own
CreatePixmap() instead, which does the CreateDIBSection(), once.

Testcase: glxgears or foobillard with direct swrast
Testcase: scrolling in a full-screen xterm

v2:
Fix handling of RENDER Scratch Pixmaps
(A problem easily shown with gitk or emacs)

v3:
Note that we don't own screen pixmap to release in DestroyPixmap
Log if unimplemented slow-path ever gets hit