x11_common: fix some problems with window dragging

There were some bad interactions with the OSC.

For one, dragging the OSC bar, and then moving the mouse outside of the
OSC (while mouse button still held) would suddenly initiate window
dragging. This was because win_drag_button1_down was not reset when
sending a normal mouse event, which means the window dragging code can
become active even after we've basically decided that the preceding
click didn't initiate window dragging.

Second, dragging the window and clicking on the OSC bar after that did
nothing. This was because no mouse button up event was sent to the core,
even though a mouse down event was sent. So make sure the key state is
erased with MP_INPUT_RELEASE_ALL.