gallium/u_blitter: Fix depth.

gallium/u_blitter: Fix depth.

Fix the transform to make sure it doesn't disturb the depth range
of the blitted image. Set the Z coordinates of the vertices
by hand instead of relying on the transform to do it.

This is a pre-requisite to Zink always enabling depth clamping.

Fixes: 26c6640835936a77d87030ce8e90f9b9f5be783e
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16929>