blorp: Handle the RGB workaround more like other workarounds

The previous version was sort-of strapped on in that it just adjusted
the blit rectangle and trusted in the fact that we would use texelFetch
and round to the nearest integer to ensure that the component positions
matched.  This new version, while slightly more complicated, is more
accurate because all three components end up with exactly the same
dst_pos and so they will get interpolated and sampled at the same
texture coordinate.  This makes the workaround suitable for using with
scaled blits.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
(cherry picked from commit 293b8de1)