clutter: Add a flag to disable heuristic max render time

Debugging purposes: allows to check if frame drops are caused by
heuristic max render time or if they are there even with the old
behavior.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1762>

clutter/clutter/clutter-frame-clock.c

@@ -19,6 +19,7 @@
 
 #include "clutter/clutter-frame-clock.h"
 
+#include "clutter/clutter-debug.h"
 #include "clutter/clutter-main.h"
 #include "clutter/clutter-private.h"
 #include "clutter/clutter-timeline-private.h"
@@ -307,7 +308,9 @@ clutter_frame_clock_compute_max_render_time_us (ClutterFrameClock *frame_clock)
   refresh_interval_us =
     (int64_t) (0.5 + G_USEC_PER_SEC / frame_clock->refresh_rate);
 
-  if (!frame_clock->got_measurements_last_frame)
+  if (!frame_clock->got_measurements_last_frame ||
+      G_UNLIKELY (clutter_paint_debug_flags &
+                  CLUTTER_DEBUG_DISABLE_DYNAMIC_MAX_RENDER_TIME))
     return refresh_interval_us - SYNC_DELAY_FALLBACK_US;
 
   for (i = 0; i < ESTIMATE_QUEUE_LENGTH; ++i)

clutter/clutter/clutter-main.c

@@ -130,6 +130,7 @@ static const GDebugKey clutter_paint_debug_keys[] = {
   { "continuous-redraw", CLUTTER_DEBUG_CONTINUOUS_REDRAW },
   { "paint-deform-tiles", CLUTTER_DEBUG_PAINT_DEFORM_TILES },
   { "damage-region", CLUTTER_DEBUG_PAINT_DAMAGE_REGION },
+  { "disable-dynamic-max-render-time", CLUTTER_DEBUG_DISABLE_DYNAMIC_MAX_RENDER_TIME },
 };
 
 gboolean

clutter/clutter/clutter-main.h

@@ -72,6 +72,7 @@ typedef enum
   CLUTTER_DEBUG_CONTINUOUS_REDRAW               = 1 << 6,
   CLUTTER_DEBUG_PAINT_DEFORM_TILES              = 1 << 7,
   CLUTTER_DEBUG_PAINT_DAMAGE_REGION             = 1 << 8,
+  CLUTTER_DEBUG_DISABLE_DYNAMIC_MAX_RENDER_TIME = 1 << 9,
 } ClutterDrawDebugFlag;
 
 /**