Add: [Emscripten] use "relative mouse mode" with SDL2

This mode doesn't wrap the mouse constantly, but requests SDL
to lock the mouse pointer. This is needed, as with Emscripten
you are not allowed to change the mouse poisition (only to lock
it into place).

src/gfx_type.h

@@ -143,6 +143,7 @@ struct CursorVars {
 	/* Drag data */
 	bool vehchain;                ///< vehicle chain is dragged
 
+	void UpdateCursorPositionRelative(int delta_x, int delta_y);
 	bool UpdateCursorPosition(int x, int y, bool queued_warp);
 
 private: