gallium: added draw_set_mrd() function to fix polygon offset

The Minimum Resolvable Depth factor depends on the driver and can't just
be computed from the number of Z buffer bits.
Glean's polygon offset test now passes with softpipe.
Still need to determine the MRD factor for other gallium drivers, if they use
the draw module's polygon offset stage...

1 files changed, 2 insertions, 0 deletions

diff --git a/src/gallium/auxiliary/draw/draw_private.h b/src/gallium/auxiliary/draw/draw_private.h
index 37c4c87f87..a16b45d340 100644
--- a/src/gallium/auxiliary/draw/draw_private.h
+++ b/src/gallium/auxiliary/draw/draw_private.h
@@ -172,6 +172,8 @@ struct draw_context
 
    boolean force_passthrough; /**< never clip or shade */
 
+   double mrd;  /**< minimum resolvable depth value, for polygon offset */
+
    /* pipe state that we need: */
    const struct pipe_rasterizer_state *rasterizer;
    struct pipe_viewport_state viewport;