tablet: move tablet tool change processing to tablet_flush

Unlike virtually everything else, the tablet tool was processed at the time
the event was read rather than when the subsequent EV_SYN came in. This causes
difficulties with tablets that send the wrong BTN_TOOL_PEN events.

Moving the tool change processing to tablet_flush() makes the injection of the
BTN_TOOL_PEN event a lot easier, simply flipping the matching bit does the
job. It also makes it easier to ignore duplicate tool updates like we've seen
in #259.

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