Alternate CopyPixels path based on get/put_tile().

For some drivers (like Xlib) it's not possible to treat the front/back color
buffers as pipe_regions.  So pipe->region_copy() won't work.  Added a new
state tracker field indicating if we can use regions for colorbuffer accesses.
This should probably be re-considered someday...

src/mesa/state_tracker/st_context.c

@@ -90,6 +90,7 @@ struct st_context *st_create_context( GLcontext *ctx,
 
    st->ctx->VertexProgram._MaintainTnlProgram = GL_TRUE;
 
+   st->haveFramebufferRegions = GL_TRUE;
 
 #if 0
    st_init_cb_clear( st );