Add project setting for agile input event flushing

If enabled, key/touch/joystick events will be flushed just before every idle and physics frame.

Enabling this can greatly improve the responsiveness to input, specially in devices that need to run multiple physics frames per each idle frame, because of not being powerful enough to run at the target frame rate.

This will only work for platforms using input buffering (regardless event accumulation). Currenly, only Android does so, but could be implemented for iOS in an upcoming PR.

main/main.h

@@ -42,6 +42,7 @@ class Main {
 	static uint32_t frame;
 	static bool force_redraw_requested;
 	static int iterating;
+	static bool agile_input_event_flushing;
 
 public:
 	static bool is_project_manager();