winspool.drv: Use CP_UNIXCP instead of CP_ACP for data that is going to or coming from system library functions.

Everywhere it's used a regular checkGLcall will do just as well.

This allows us to drop the load time conversion and the clear
readback hack and replaces it with a color fixup in the fixed
function pipeline replacement.

Based on a patch by Stefan Dösinger. This is more flexible, and allows
the shader backend implementation to be simpler, since it doesn't have
to know about specific formats. The next patch makes use of this.