For `Drag` mousebinds, `pressed_in_context` is set by
`cursor_process_button_press()` and cleared by `cursor_process_motion()`
which runs actions bound to them. However, when `cursor_process_motion()`
is called while interactive move/resize, it doesn't clear
`pressed_in_context` due to the early-return and the `Drag` mousebinds are
unexpectedly executed on another call to `cursor_process_motion()` after
the interactive move/resize is finished by button release, even when the
button is not pressed.
So this commit fixes it by always clearing `pressed_in_context` on button
releases.
This gives instant feedback when changing cursor theme or size.
It only works for server side cursors or clients using the
cursor-shape protocol.
Fixes: #1619
