Rasterizers are now in sync with engine

The rasterisers (both GLES3 and GLES2) were calculating their own frame delta time
This fix lets the rasterizers get the frame delta through the draw call
That way any regulations to the frame step from the main script will not cause particle systems to process at a different step than the rest of the Engine.

Remove unused rasterizer storage variable

frame.prev_tick variable were not used anywhere and has been removed

servers/visual_server.h

@@ -939,7 +939,7 @@ public:
 
 	/* EVENT QUEUING */
 
-	virtual void draw(bool p_swap_buffers = true) = 0;
+	virtual void draw(bool p_swap_buffers = true, double frame_step = 0.0) = 0;
 	virtual void sync() = 0;
 	virtual bool has_changed() const = 0;
 	virtual void init() = 0;