gpu: Add GSK_DEBUG=occlusion

Draws a semi-transparent white overlay over all regions that have
been chosen for occlusion.

gsk/gpu/gskgpunodeprocessor.c

@@ -4101,6 +4101,17 @@ gsk_gpu_node_processor_render (GskGpuFrame                 *frame,
           gsk_gpu_node_processor_add_node (&self, node);
           do_culling = FALSE;
         }
+      else if (GSK_DEBUG_CHECK (OCCLUSION))
+        {
+          gsk_gpu_node_processor_sync_globals (&self, 0);
+          gsk_gpu_color_op (self.frame,
+                            GSK_GPU_SHADER_CLIP_NONE,
+                            self.ccs,
+                            1.0,
+                            &self.offset,
+                            &GRAPHENE_RECT_INIT(0, 0, 10000, 10000),
+                            &GDK_COLOR_SRGB (1.0, 1.0, 1.0, 0.6));
+        }
 
       gsk_gpu_render_pass_end_op (frame,
                                   target,

gsk/gskdebug.c

@@ -15,6 +15,7 @@ static const GdkDebugKey gsk_debug_keys[] = {
   { "staging", GSK_DEBUG_STAGING, "Use a staging image for texture upload (Vulkan only)" },
   { "offload-disable", GSK_DEBUG_OFFLOAD_DISABLE, "Disable graphics offload" },
   { "cairo", GSK_DEBUG_CAIRO, "Overlay error pattern over Cairo drawing (finds fallbacks)" },
+  { "occlusion", GSK_DEBUG_OCCLUSION, "Overlay highlight over areas optimized via occlusion culling" },
 };
 
 static guint gsk_debug_flags;

gsk/gskdebugprivate.h

@@ -18,6 +18,7 @@ typedef enum {
   GSK_DEBUG_STAGING               = 1 << 10,
   GSK_DEBUG_OFFLOAD_DISABLE       = 1 << 11,
   GSK_DEBUG_CAIRO                 = 1 << 12,
+  GSK_DEBUG_OCCLUSION             = 1 << 13,
 } GskDebugFlags;
 
 #define GSK_DEBUG_ANY ((1 << 13) - 1)