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.

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: Split from the core gallium commit, drop some unnecessary code related
    to glBlitFramebuffer(), fix a crash with clears before state has been
    bound.