Enable parallel painting on OSX and cap the amount of paint workers to 4. (bug 1430793, r=milan)

--HG--
extra : rebase_source : 9d5c853f84b160819540cc062fb84f4bea8fcb02

gfx/layers/PaintThread.cpp

@@ -137,13 +137,10 @@ PaintThread::CalculatePaintWorkerCount()
 
   int32_t workerCount = gfxPrefs::LayersOMTPPaintWorkers();
 
-  // If not manually specified, default to (cpuCores * 3) / 4
+  // If not manually specified, default to (cpuCores * 3) / 4, and clamp
+  // between 1 and 4. If a user wants more, they can manually specify it
   if (workerCount < 1) {
-    workerCount = std::max((cpuCores * 3) / 4, 1);
-
-    if (workerCount > 32) {
-      workerCount = 32;
-    }
+    workerCount = std::min(std::max((cpuCores * 3) / 4, 1), 4);
   }
 
   return workerCount;

modules/libpref/init/all.js

@@ -5935,5 +5935,9 @@ pref("layers.omtp.enabled", true);
 #else
 pref("layers.omtp.enabled", false);
 #endif
-pref("layers.omtp.release-capture-on-main-thread", false);
+#if defined(XP_MACOSX)
+pref("layers.omtp.paint-workers", -1);
+#else
 pref("layers.omtp.paint-workers", 1);
+#endif
+pref("layers.omtp.release-capture-on-main-thread", false);