Bug 1547775: Enable double buffering on D3D11 compositors. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D29243

--HG--
extra : source : d5b295babcb578cb412f2c1cb056d0eaa817564d
extra : intermediate-source : c67ffb0e9865ebcc13e4dfb51afde9f0e94163c5

modules/libpref/init/all.js

@@ -5170,6 +5170,10 @@ pref("gfx.direct2d.destroy-dt-on-paintthread", true);
 pref("gfx.direct3d11.enable-debug-layer", false);
 pref("gfx.direct3d11.break-on-error", false);
 
+// Prefer flipping between two buffers over copying from our back buffer
+// to the OS.
+pref("gfx.direct3d11.use-double-buffering", true);
+
 pref("layers.prefer-opengl", false);
 #endif