Add a function to transform screen points to projected space.

This is accessible from QML land.
Fix screenMatrix() returning wrong matrix.
Make the QML label follow mouse and display points.
Fix getting mouse events.

Since we have a full-size mouse area, the QQuickView is now swallowing
mouse events. Need to disable checking for events being accepted, at
least for now.
Some clean-ups to PointLayer now that more QML works.

Layer still doesn't fully work, but things are closer.
WIP: Trying to get points in QML displayed using our Transform.