default pressure when not specified for touch/pen input

[gsnedders]

We have tests in WPT that assert a non-zero pressure when pointerdown gets fired for touch/pen input, but nothing actually defines how much pressure they should have; it seems sensible to assert that the digitizer has actually noticed something (and thus has non-zero pressure).

While this arguably falls into the "Perform implementation-specific action dispatch steps", it would probably be better to define the default pressure if not specified in WebDriver.