Ability to discern between touch and pen/stylus input

[Bloodninj]

It might be useful to be able to tell whether a touch event was caused by a regular finger touch or by a pen/stylus input. An example use case would be to allow for programs to use only pens for drawing, while ignoring regular touch on a canvas (for palm rejection, for example, or using regular touch to pan the drawing canvas instead).

I originally wanted to implement this in HaxeFlixel (https://github.com/HaxeFlixel/flixel/pull/2588) assuming that support existed in Lime and was not being passed down or exposed, and assumed that I could determine the input type based on reported pressure values. However, this seems to vary between browsers and even platforms:

• Chromium-based browsers (on Windows, at least) report that regular touch has a pressure of 1.0, whereas Firefox reports 0
• iOS Safari reports 0 for regular touch, and while it correctly reports pressure from an Apple Pencil stylus
  • It also seems to allow 0 as a valid pressure value from pen input, usually for extremely light taps or the end of a pen touch.

Because of these cross-browser and -platform differences, it would be easier to determine the input device getting it straight from the root touch event as reported by Lime.