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)

modules/libpref/init/all.js

@@ -4505,7 +4505,16 @@ pref("network.tcp.keepalive.probe_count", 4);
 #endif
 
 // Whether to disable acceleration for all widgets.
-pref("layers.acceleration.disabled", false);
+#if defined(XP_UNIX) && !defined(XP_MACOSX)
+// On Linux this is disabled by default for known issues with "free" drivers
+pref("layers.acceleration.enabled", false);
+#else
+pref("layers.acceleration.enabled", true);
+#endif
+// Whether to force acceleration on, ignoring blacklists.
+// This requires layers.acceleration.enabled to be set to true
+pref("layers.acceleration.force", false);
+
 // Preference that when switched at runtime will run a series of benchmarks
 // and output the result to stderr.
 pref("layers.bench.enabled", false);
@@ -4514,18 +4523,6 @@ pref("layers.bench.enabled", false);
 pref("layers.gpu-process.dev.enabled", true);
 #endif
 
-// Whether to force acceleration on, ignoring blacklists.
-#ifdef ANDROID
-// bug 838603 -- on Android, accidentally blacklisting OpenGL layers
-// means a startup crash for everyone.
-// Temporarily force-enable GL compositing.  This is default-disabled
-// deep within the bowels of the widgetry system.  Remove me when GL
-// compositing isn't default disabled in widget/android.
-pref("layers.acceleration.force-enabled", true);
-#else
-pref("layers.acceleration.force-enabled", false);
-#endif
-
 pref("layers.acceleration.draw-fps", false);
 
 // Enable DEAA antialiasing for transformed layers in the compositor