uxa: use GCPtr instead of GC *

Xserver includes have explicit pointer types for quite all kind of structs (at least those used by drivers), which are used all over the Xserver. Thus it's much cleaner to use those everywhere.

This commit also clears the road to fix a horrible nightmare of hacks just needed to circumvent naming clashes between Xserver and Xlib (affecting all DDXes that are painting on another Xserver): we can simply rename Xserver's own "GC" type to "GCRec" (the usual naming convention here) and so no trouble with Xlib's "GC" type anymore. Once this has landed, we're free to do that without breaking this driver.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net