drm/state-propose: Remove special casing for cursor plane

Previously we assumed that cursor planes occluded nothing and would
always be blended, but overlay and scanout planes would always occlude
what's behind them. This is not actually true, as we can support alpha
blending on any kind of plane type now.

Remove the special case, which might hopefully fix some weird display
issues along the way. (Noticed by inspection.)

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/backend-drm/state-propose.c

@@ -929,14 +929,6 @@ drm_output_propose_state(struct weston_output *output_base,
 			drm_debug(b, "\t\t\t[plane] next zpos to use %"PRIu64"\n",
 				      current_lowest_zpos);
 
-			/* If we have been assigned to an overlay or scanout
-			 * plane, add this area to the occluded region, so
-			 * other views are known to be behind it. The cursor
-			 * plane, however, is special, in that it blends with
-			 * the content underneath it: the area should neither
-			 * be added to the renderer region nor the occluded
-			 * region. */
-			if (ps->plane->type != WDRM_PLANE_TYPE_CURSOR) {
 			if (!weston_view_is_opaque(ev, &clipped_view))
 				pixman_region32_intersect(&clipped_view,
 							  &clipped_view,
@@ -948,8 +940,6 @@ drm_output_propose_state(struct weston_output *output_base,
 					      &clipped_view);
 
 			pixman_region32_fini(&clipped_view);
-
-			}
 			continue;
 		}
 
@@ -964,15 +954,21 @@ drm_output_propose_state(struct weston_output *output_base,
 			goto err_region;
 		}
 
+		/* clipped_view contains the area that's going to be visible
+		 * on screen; add this to the renderer region */
 		pixman_region32_union(&renderer_region,
 				      &renderer_region,
 				      &clipped_view);
 
+		/* Opaque areas of our clipped view occlude areas behind it;
+		 * however, anything not in the opaque region (which is the
+		 * entire clipped area if the whole view is known to be
+		 * opaque) does not necessarily occlude what's behind it, as
+		 * it could be alpha-blended. */
 		if (!weston_view_is_opaque(ev, &clipped_view))
 			pixman_region32_intersect(&clipped_view,
 						  &clipped_view,
 						  &ev->transform.opaque);
-
 		pixman_region32_union(&occluded_region,
 				      &occluded_region,
 				      &clipped_view);