Enable xrender by default.

[Peacock365]

Resolves perf regressions over remote X.

[grahamperrin]

From #553 (comment):

If there's a choice of rendering back-ends (e.g. XRender | OpenGL 2.0 | OpenGL 3.1) for compositing, then should the updated Skia be tested against all back-ends?

• XRender is probably irrelevant: disabled due to unpredictable performance and interference with OpenGL
• Skia takes advantage of OpenGL compositing via gfx.canvas.azure.accelerated=true (aka SkiaGPU)

I see gfx.xrender.enabled false by default with e.g. www/firefox 60.0.2,1

@jbeich please, do you know, is the interference specific to FreeBSD?

[jbeich]

@grahamperrin, OpenGL compositing is layers.acceleration.force-enabled=true. Try benchmarking e.g., via HTML5 games.