Because vc4 can control the order that tiles are rasterized in, we can use
it to implement overlapping blits using normal drawing and
GL_ARB_texture_barrier, as long as we can tell the kernel what order to
render the tiles in.

This commit introduces the core gallium support, vc4 changes will follow.

v2: Fix on the simulator.
v3: Add the cap (disabled) to other drivers, add rst docs for the cap.
v4: Rebase on PIPE_CAP_TGSI_ANY_REG_AS_ADDRESS
v5: Drop vc4 changes from this commit, for clarity.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v3)