[X11] fix cursor jumping when grabbed

[djdisodo]

Description i found that cursor jumps randomly when you move your cursor only when you grabbed

it occurs because you set last_x and last_y while another MotionNotify event is in the queue, possibly an event before XWarpPointer applied

this patch make sure there is no MotionNotify event in the queue before setting last_x and last_y, warping cursor and sending mouse move