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_driver.h

@@ -456,6 +456,10 @@ public:
 	// Retrieve the format used by the swap chain's framebuffers.
 	virtual DataFormat swap_chain_get_format(SwapChainID p_swap_chain) = 0;
 
+	// Tells the swapchain the max_fps so it can use the proper frame pacing.
+	// Android uses this with Swappy library. Some implementations or platforms may ignore this hint.
+	virtual void swap_chain_set_max_fps(SwapChainID p_swap_chain, int p_max_fps) {}
+
 	// Wait until all rendering associated to the swap chain is finished before deleting it.
 	virtual void swap_chain_free(SwapChainID p_swap_chain) = 0;