loader_dri3/glx/egl: Optionally use a blit context for blitting operations

loader_dri3/glx/egl: Optionally use a blit context for blitting operations

The code was relying on us always having a current context for client local
image blit operations. Otherwise the blit would be skipped. However,
glxSwapBuffers, for example, doesn't require a current context and that was a
common problem in the dri1 era. It seems the problem has resurfaced with dri3.

If we don't have a current context when we want to blit, try creating a private
dri context and maintain a context cache of a single context.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>