llvmpipe: Fix alpha-to-coverage without dithering

Implementing alpha-to-coverage dithering broke the non-dithering case.
(Discovered by accident, not really a big deal since it's almost always
enabled and can only be disabled by using a Nvidia GL extension, and
can't be disabled with Vulkan.)

Fixes: ad4635d6ef
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33586>

src/gallium/drivers/llvmpipe/lp_bld_blend.c

@@ -321,6 +321,7 @@ lp_build_alpha_to_coverage(struct gallivm_state *gallivm,
                            struct lp_type type,
                            struct lp_build_mask_context *mask,
                            LLVMValueRef alpha,
+                           bool dither,
                            bool do_branch)
 {
    struct lp_build_context bld;
@@ -329,7 +330,7 @@ lp_build_alpha_to_coverage(struct gallivm_state *gallivm,
 
    lp_build_context_init(&bld, gallivm, type);
 
-   alpha_ref_value = lp_build_const_vec(gallivm, type, 0);
+   alpha_ref_value = lp_build_const_vec(gallivm, type, dither ? 0 : 0.5);
 
    test = lp_build_cmp(&bld, PIPE_FUNC_GREATER, alpha, alpha_ref_value);
 

src/gallium/drivers/llvmpipe/lp_bld_blend.h

@@ -104,6 +104,7 @@ lp_build_alpha_to_coverage(struct gallivm_state *gallivm,
                            struct lp_type type,
                            struct lp_build_mask_context *mask,
                            LLVMValueRef alpha,
+                           bool dither,
                            bool do_branch);
 
 #endif /* !LP_BLD_BLEND_H */

src/gallium/drivers/llvmpipe/lp_state_fs.c

@@ -1150,6 +1150,7 @@ generate_fs_loop(struct gallivm_state *gallivm,
          if (!key->multisample) {
             lp_build_alpha_to_coverage(gallivm, type,
                                        &mask, alpha,
+                                       key->blend.alpha_to_coverage_dither,
                                        (depth_mode & LATE_DEPTH_TEST) != 0);
          } else {
             lp_build_sample_alpha_to_coverage(gallivm, type, key->coverage_samples, num_loop,