backends/x11: Store eraser and stylus tools separately
Our hashtable stores tools by the serial but our stylus tool and eraser tool share the same serial - they only differ by the tool type.
This results in only one tool being created and this tool re-used for the other type tool. Fun side-effects of this are that the stylus ends up using the eraser pressure curve (or vice versa).
Hack around this by bit-flipping the serial for the eraser to make it distinct - this is the only place we need to wrorry .
Closes #1884 (closed)
cc @jigpu