Fix intermittent cursor ghosting when crossing between monitors

Live report: the real cursor sometimes leaves a brief ghost behind right
after moving between monitors. The bare-metal render loop already forces
a full repaint (ages = [0, 0]) on a workspace switch or any window move/
resize/open/close/restack, both added earlier for the same underlying gap:
the damage tracker's own element diffing doesn't always catch a vacated
region on its own. Neither reset noticed the pointer leaving one monitor
for another - no window moved, no workspace changed - so that head's own
vacated cursor-sized region was left entirely to the tracker's diffing,
intermittently.

Adds UdevState::last_cursor_head, compared each frame the same way the
other two resets are; only the head the pointer just left gets forced back
to ages = [0, 0] (the newly-entered head draws a genuinely new element
there and diffs correctly on its own).

0 files changed, 0 insertions, 0 deletions