panfrost: do not allow skipping of fragment shader when alpha-to-coverage

When alpha to coverage is enabled we cannot allow the fragment shader to
be skipped, because the calculated alpha result is essentially a side
effect (it can cause some samples to be discarded).

This brings the OpenGL driver in line with panvk, which already has this
check in its version of fs_required().

Signed-off-by: Eric R. Smith <eric.smith@collabora.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>

Cc: mesa-stable
(cherry picked from commit 3ce6bcb9e8)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39003>

.pick_status.json

@@ -14,7 +14,7 @@
         "description": "panfrost: do not allow skipping of fragment shader when alpha-to-coverage",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/gallium/drivers/panfrost/pan_cmdstream.h

@@ -225,6 +225,11 @@ panfrost_fs_required(struct panfrost_compiled_shader *fs,
    if (fs->info.fs.sidefx)
       return true;
 
+   /* If alpha to coverage is enabled we need to execute, as
+    * writing the pixel can modify occlusion query results. */
+   if (blend->base.alpha_to_coverage)
+      return true;
+
    /* Using an empty FS requires early-z to be enabled, but alpha test
     * needs it disabled. Alpha test is only native on Midgard, so only
     * check there.