gallium/u_blitter: work around broken sample shading in llvmpipe and zink

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13602>

src/gallium/auxiliary/util/u_blitter.c

@@ -349,6 +349,13 @@ struct blitter_context *util_blitter_create(struct pipe_context *pipe)
    return &ctx->base;
 }
 
+void util_blitter_sample_shading_force_off(struct blitter_context *blitter)
+{
+   struct blitter_context_priv *ctx = (struct blitter_context_priv*)blitter;
+
+   ctx->has_sample_shading = false;
+}
+
 void *util_blitter_get_noop_blend_state(struct blitter_context *blitter)
 {
    struct blitter_context_priv *ctx = (struct blitter_context_priv*)blitter;

src/gallium/auxiliary/util/u_blitter.h

@@ -154,6 +154,7 @@ struct blitter_context *util_blitter_create(struct pipe_context *pipe);
  */
 void util_blitter_destroy(struct blitter_context *blitter);
 
+void util_blitter_sample_shading_force_off(struct blitter_context *blitter);
 void util_blitter_cache_all_shaders(struct blitter_context *blitter);
 void *util_blitter_get_noop_blend_state(struct blitter_context *blitter);
 void *util_blitter_get_noop_dsa_state(struct blitter_context *blitter);

src/gallium/drivers/llvmpipe/lp_context.c

@@ -268,6 +268,7 @@ llvmpipe_create_context(struct pipe_screen *screen, void *priv,
       goto fail;
    }
 
+   util_blitter_sample_shading_force_off(llvmpipe->blitter);
    /* must be done before installing Draw stages */
    util_blitter_cache_all_shaders(llvmpipe->blitter);
 

src/gallium/drivers/zink/zink_context.c

@@ -4153,6 +4153,8 @@ zink_context_create(struct pipe_screen *pscreen, void *priv, unsigned flags)
    if (!ctx->blitter)
       goto fail;
 
+   util_blitter_sample_shading_force_off(ctx->blitter);
+
    ctx->gfx_pipeline_state.shader_keys.last_vertex.key.vs_base.last_vertex_stage = true;
    ctx->last_vertex_stage_dirty = true;
    ctx->gfx_pipeline_state.shader_keys.key[PIPE_SHADER_VERTEX].size = sizeof(struct zink_vs_key_base);