Add Swappy & Pre-Transformed Swapchain

- Adds Swappy for Android for stable frame pacing
- Implements pre-transformed Swapchain so that Godot's compositor is in
charge of rotating the screen instead of Android's compositor
(performance optimization for phones that don't have HW rotator)

============================

The work was performed by collaboration of TheForge and Google. I am
merely splitting it up into smaller PRs and cleaning it up.

Changes from original PR:

- Removed "display/window/frame_pacing/android/target_frame_rate" option
to use Engine::get_max_fps instead.
- Target framerate can be changed at runtime using Engine::set_max_fps.
- Swappy is enabled by default.
- Added documentation.
- enable_auto_swap setting is replaced with swappy_mode.

servers/rendering/rendering_device.h

@@ -1421,6 +1421,8 @@ public:
 	Error initialize(RenderingContextDriver *p_context, DisplayServer::WindowID p_main_window = DisplayServer::INVALID_WINDOW_ID);
 	void finalize();
 
+	void _set_max_fps(int p_max_fps);
+
 	void free(RID p_id);
 
 	/****************/