The following changes are made in this patch:
- Fix tracking of pressed buttons. Previously, pressing two buttons and
then releasing one would make st think no buttons are pressed, which
in particular broke MODE_MOUSEMOTION.
- Always send the lowest-numbered pressed button on motion events; when
no button is pressed for a motion event in MODE_MOUSEMANY, then send
a release. This matches the behaviour of xterm. (Previously, st sent
the most recently pressed button in the motion report.)
- Remove UB (?) access to potentially inactive struct member
e->xbutton.button of XEvent union.
- Fix (unlikely) possibility of overflow for large button numbers.
The one discrepancy I found between st and xterm is that xterm sometimes
encodes buttons with large numbers (>5) strangely. E.g., xterm reports
presses of buttons 8 and 9 as releases, whereas st properly (?) encodes
them as presses.