Implement keyboard grab support for Windows

This is implemented via a low-level keyboard hook. Unfortunately, this is
rather invasive, but it's how Microsoft recommends that it be done [0].
We want to do as little as possible in the hook, so we only intercept a few
crucial modifier keys there, while leaving other keys to the normal event
processing flow.

We will only install this hook if SDL_HINT_GRAB_KEYBOARD=1, which is not
the default. This will reduce any compatibility concerns to just the SDL
applications that explicitly ask for this behavior.

We also remove the hook when the grab is terminated to ensure that we're
not unnecessarily staying involved in key event processing when it's not
required anymore.

4 files changed