Add hidden pref for forcing readback-based compositing for WebGL.
Created attachment 608903 [details] [diff] [review] fix? Here's a first attempt. I'll try it out shortly.
Created attachment 645136 [details] [diff] [review] force readback pref for non-omtc OMTC version needs OMTC perf checking, which I forgot how to do.
Created attachment 652226 [details] [diff] [review] add support for forcing OMTC readback Sorry, I forgot to make this a patch on top of the other one. :(