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.cpp

@@ -6518,6 +6518,12 @@ void RenderingDevice::finalize() {
 	ERR_FAIL_COND(reverse_dependency_map.size());
 }
 
+void RenderingDevice::_set_max_fps(int p_max_fps) {
+	for (const KeyValue<DisplayServer::WindowID, RDD::SwapChainID> &it : screen_swap_chains) {
+		driver->swap_chain_set_max_fps(it.value, p_max_fps);
+	}
+}
+
 RenderingDevice *RenderingDevice::create_local_device() {
 	RenderingDevice *rd = memnew(RenderingDevice);
 	rd->initialize(context);