The touchpad_2fg_scroll_initially_diagonal test would semi-reliably fail under
valgrind but succeed otherwise. Cause was that on some devices, the initial
diagonal movement wasn't diagonal enough and closer to a horizontal movement.
This was fine on normal runs, but under valgrind we'd hit the "active
threshold" time limit and lock to horizontal scrolling, ditching the remaining
events and failing the test.

Fix this by calculating the scroll vector based on the device's width/height
ratio and go "more diagonal" on the initial vector.

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