Examples: fix touch handling in painting examples

On macOS, we don't deliver a press event for the first press on the
track pad (Qt::WA_TouchPadAcceptSingleTouchEvents is not set by default,
so Qt doesn't deliver a single-press on the track pad as a touch event -
that makes some sense or at least maintains compatibility). Because of
that, point 0 is never added to the finger-mapping hash.

When point 0 is then released, we didn't check if we found a valid
iterator for that point ID, and the example crashed.

Fix this by checking that we have a valid iterator before dereferencing,
and by handling Stationary events in the same way as pressed (add the
point to the mapping if it's not already there).

Pick-to: 6.7 6.6 6.5
Fixes: QTBUG-110266
Change-Id: I32337b801aaabf9b821a97ddc15ad78747b5e6a2
Reviewed-by: Jan Arve Sæther <[email protected]>

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