gpu: add sync mode config

This allows the additional OFF mode which has the client manage its own
frame rate target without the use of vsync. On lower end systems which
have a hard time delivering a frame in the vsync interval this can give
a noticible performance improvement since it doesn't stall the cpu
waiting for the next vblank when a frame is skipped

runelite-api/src/main/java/net/runelite/api/Client.java

@@ -1953,6 +1953,7 @@ public interface Client extends GameEngine
 	ClanSettings getClanSettings(int clanId);
 
 	void setUnlockedFps(boolean unlock);
+	void setUnlockedFpsTarget(int fps);
 
 	/**
 	 * Gets the ambient sound effects