Detect eraser pen input rather than treating it as mouse input

[5Mixer]

Currently only the 'pen' side of tablet pen input is detected. The eraser side is treated as mouse input. Reusing the existing Pen api as much as possible makes sense - the same events with x, y, and pressure are relevant. Alternative methods for the eraser could be created, or perhaps an additional eraser boolean could be passed.

Relevant Kha issue: Kode/Kha#1240

[RobDangerous]

There's a merged pull-request. There's a closed Kha-issue. That probably means we're done here.