avoid draw buffer -> display buffer copy

[monfera]

General WebGL speedup (currently done for 2D plots only) that can help low-end platforms such as current MacBook and mobile platforms that have large display buffers (high resolution)

[etpinard]

Hmm. How does this affect to-image exports?

If I recall correctly preserveDrawingBuffer has to be turned on for to-image to work correctly (I might be wrong though).

[monfera]

Thanks for highlighting this potential issue. For the record, we decided to check effects and if needed, apply the synchronous export approach which doesn't need preserveDrawingBuffer to be true (buffer isn't cleared yet).

[monfera]

... I was curious since further tests showed that Chrome, Safari and FF all seem to export fine with this flag set to false. I believe it's because, for the 2D plots at least (for which this PR applies) there's already an intention to be synchronous:

1. The render loop is temporarily disabled
2. There is a single draw call performed synchronously just before the export

With this and as discussed, I'd leave the call with your further investigation and decision. On reasonable hardware this PR doesn't seem mandatory for a decent framerate even with the upcoming pointcloud but probably helps weaker notebooks and mobile.

[etpinard]

After some testing in browsers and in our nw.js image exporter, this PR has no unwanted side-effects.

Merging for perf 💊

Thanks @monfera !