Some small improvements relating to rendering:
- Add a debug option: `-debug.pixel-read-period`. This can be used to measure the performance impact of checking the pointer location on different hardware. [On my development box, it makes no difference to performance.] (Closes#5490).
- Unbind pixel pack buffers after each use. This is recommended practice. It has no performance impact on my machine, and allows SpectorJS to run (`-debug.enable-spector`). (Closes#5941).
Also, simplify the profiling CLI: the `profile.load-profile` and `profile.save-profile` options have been renamed to `profile.load`/`profile.save`; `profile.save` now has a default filename, so you can capture a profile at any time in Electron with Ctrl+Alt+P and it will be written to `profile.json`.
