I've implemented support for exposing the pixel delta of XInput 2 wheel
events to Lisp code.  With GC turned off, utilizing a modified
pixel-scroll.el with this support results in a quite pleasant
experience, but pixel-scroll still GCs too much to be useful.

Pixel based scrolling is a popular feature in modern X applications, and
does make life a lot easier in many cases.  The existing support is
inadequate as it cannot utilize the detailed delta information available
from modern touch-based scroll wheels, which this patch enables.

The XInput 2 support isn't the best from a code duplication POV, but the
behavior of the XInput 2 code often differs in subtle ways from that of
the Core Input code, and trying to combine the two together would result
in a more catastrophic mess than leaving them apart.

Obviously, here is the original XInput 2 support patch: