test: change tablet coords to doubles and pass the pointer through

test: change tablet coords to doubles and pass the pointer through

All other motion/touch/... coords are already doubles so let's follow
suite here. And passing a pointer into the custom handlers
means we can modify x/y slightly and return false, leaving the rest up
to the generic event handling code.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>