Add editor.gpuDisabled option to prevent React editor from using layers

The following issues fixed by this PR have to do with using the GPU for rendering.

* Fixes #2511. Font rendering is a bit different on OS X when the GPU is employed due to issues with subpixel anti-aliasing, so this allows it to be disabled at the cost of performance if the user truly can't stand it. If we get a lot of complaints, we may want to auto-disable the GPU on non-retina displays.

* Fixes #2769. It seems like there are bugs on Linux with certain graphics drivers. A quick workaround is just disabling the GPU entirely. Hopefully these issues will be addressed as Chromium evolves.

![gpu-disabled](https://cloud.githubusercontent.com/assets/1789/3447052/1d6c4bd2-0149-11e4-9ee0-3048230eb77c.gif)