This expands input events to represent sub-pixel mouse positions, devices responsible for generating events, and raw
device-oriented events. The X11 back end is refactored to make full use of the new expressiveness. Other backends have
had new functionality minimally stubbed out, save for the macos backend which already supports sub-pixel mouse
positions.
Making applications track modifier keys results in unnecessary work for
consumers, it's error prone, and it turns out to have unavoidable bugs.
For example, alt-tabbing with x11 results in the alt modifier state
getting stuck.
To resolve these problems, this patch adds a Mods value to the keyboard
input event.
Based on this patch: d287fa96e3