Issue #1360 - Part 1: Simplify layers acceleration prefs.

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)

gfx/thebes/gfxPlatformMac.h

@@ -81,9 +81,6 @@ public:
     // lower threshold on font anti-aliasing
     uint32_t GetAntiAliasingThreshold() { return mFontAntiAliasingThreshold; }
 
-protected:
-    bool AccelerateLayersByDefault() override;
-
 private:
     virtual void GetPlatformCMSOutputProfile(void* &mem, size_t &size) override;