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/gfxPlatformGtk.h

@@ -132,10 +132,6 @@ public:
       return true;
     }
 
-    bool AccelerateLayersByDefault() override {
-      return true;
-    }
-
 #ifdef GL_PROVIDER_GLX
     already_AddRefed<mozilla::gfx::VsyncSource> CreateHardwareVsyncSource() override;
 #endif