gallium: make BlendCoherent usable from gallium drivers

Signed-off-by: Amber Amber <amber@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21161>

src/gallium/include/pipe/p_state.h

@@ -376,6 +376,7 @@ struct pipe_blend_state
    unsigned alpha_to_one:1;
    unsigned max_rt:3;            /* index of max rt, Ie. # of cbufs minus 1 */
    unsigned advanced_blend_func:4;
+   unsigned blend_coherent:1;
    struct pipe_rt_blend_state rt[PIPE_MAX_COLOR_BUFS];
 };
 

src/mesa/state_tracker/st_atom_blend.c

@@ -263,6 +263,10 @@ st_update_blend( struct st_context *st )
       blend->rt[i].colormask = colormask;
    }
 
+   if (ctx->Color._AdvancedBlendMode != BLEND_NONE) {
+      blend->blend_coherent = ctx->Color.BlendCoherent;
+   }
+
    if (ctx->Color.ColorLogicOpEnabled) {
       /* logicop enabled */
       blend->logicop_enable = 1;