Snapshot rotation during brush resizing (alternate action)

This patch attempts to fix the behavior described at the end of the video in !2314 (comment 1172727).

When resizing the brush with shift-drag, the outline will reset to the default angle, i.e. it will ignore the current pen tilt. This change will copy the initial pen event, including the tilt info, and reuse it during the whole alternate action.

NOTE I'm not entirely sure what's the correct way to handle event wrapper ownership. I used std::optional to dodge the issue of creating an "empty" wrapper.

PS. I was tempted to remove the commented out //m_brushResizeCompressor.start(newSize);, but I'm seeing a preference to avoid non-essential changes in a patch, so I left it be.

FYI @davidrevoy