Only mention OpenGL accelerated canvas flag when it's relevant, and rename
the entry to be more descriptive, to prevent confusion for users looking
at Troubleshooting Information.
This gets rid of platform-dependent hard-coded defaults, but keeps
build-time blocking if there is no GL provider (in which case layers
acceleration almost certainly won't work because it needs a GL
compositor and would likely crash without)
New prefs are
- layers.acceleration.enabled to enable HWA
- layers.acceleration.force to force it enabled (requires .enabled to be
set as well)
This is the platform part of this issue. The rest will be front-end work
(Preference UI integration and pref migration)