emscripten: Move over to using Pointer Events for all mouse, pen, and touch.

This allows us to avoid browser mouse emulation for touches, since we provide
our own anyhow.

The other option is to "prevent default" in the legacy touch event handlers we
historically used, to tell the browser not to supply emulation, but we can't
currently tell Emscripten to mark those handlers as not "passive," so as it
stands they are unable to prevent default. Using Pointer Events bypasses this
problem entirely.

Fixes #13161.

Author: icculus