24e8ba048a did not take scaling into account.
The hotspot size used pixel coordinates, the absolute coordinates were logical,
and the relative coordinates were completely wrong.
This commit makes all coordinates use logical values. If
`"float_event_coords":true` is sent in the handshake message, coordinates are
sent as floating-point values.
The "scale" field is an integer containing the scale value.
7affe5c1bd