force software fallback for drawing rectangles into offscreen memory only
on NV_ARCH_40 and up

set EXA_MIXED_PIXMAPS so we don't get pixmaps we can't handle in NVSolid()
while there shut up some related debug output and remove unused variables

for some reason my 6800 Ultra stalls when drawing *some* off-screen rectangles
as a workaround, draw them by software until I figure out what's going on

- increase offscreen pixmap alignment
- don't pretend to support overlapping pixmaps
- reshuffle some register writes
- set surface formats every time we Prepare*()
-> less garbage, almost works correctly now

fill in PrepareAccess() and FinishAcces() methods

Fix cats build.

add mostly working EXA support
tested on a GeForce 6800 Ultra in a G5
still produces occasional artifacts but good enough to run a bunch of xterms
and gtk2 applications