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.

core/config/engine.cpp

@@ -36,6 +36,7 @@
 #include "core/license.gen.h"
 #include "core/variant/typed_array.h"
 #include "core/version.h"
+#include "servers/rendering/rendering_device.h"
 
 void Engine::set_physics_ticks_per_second(int p_ips) {
 	ERR_FAIL_COND_MSG(p_ips <= 0, "Engine iterations per second must be greater than 0.");
@@ -68,6 +69,11 @@ double Engine::get_physics_jitter_fix() const {
 
 void Engine::set_max_fps(int p_fps) {
 	_max_fps = p_fps > 0 ? p_fps : 0;
+
+	RenderingDevice *rd = RenderingDevice::get_singleton();
+	if (rd) {
+		rd->_set_max_fps(_max_fps);
+	}
 }
 
 int Engine::get_max_fps() const {