blitter_context_priv::vertices is initialized at start-up, but can (and will) be overwritten by calls to the blitter since then. Since we always copy from the LOD read by the the vertex-shader, and we risk blitting from out-of-range LODs, which give incorrect results.
So let's just bite the bullet and not use the XY-codepath in the shared code. The difference is just a few writes, but then we always update all the read state as apropriate.
This fixes a case for me where the following two Khronos Confidental CTS cases are run back to back:
Fixes: 22ed1ba0 ("gallium/u_blitter: use draw_rectangle for all blits except cubemaps")