Software cursor broken with several X screens

[Pierre-Loup A. Griffais]

The DC code is broken for setups with several screens. Devs only have one pSave 
pixmap and there's no code to thrash them like p[Save|Restore]GC.

That means if you have two X screens and force SW cursor on both, the server
will end up passing a bunch of CopyAreas with mismatching screens to the driver, 
which can fail horribly if the driver does a good job abstracting screens away 
from each other.

The GC thrashing code looks a bit clunky in the first place. Can we just have an 
array with numScreens elements for the per-device GCs and pixmaps?

Thanks,
   - Pierre-Loup