This patch implements support for SDL_HINT_GRAB_KEYBOARD on Windows using a keyboard hook. I've shipped a version of this in Moonlight, but there were a lot more hoops to jump through due to the fact that I had to keep manually synchronizing SDL's modifier state because I was eating some of the keyboard events prior to them reaching SDL. The code is much simpler in SDL itself where we can just call SDL_SendKeyboardKey() and it will do the magic for us.

The only gotcha is that SetWindowHookEx() needs an HMODULE and that's challenging for us since we support DLL and static builds of SDL. Right now the code assumes that SDL2.dll is where our code will reside, but it will just gracefully fail if SDL2.dll is not loaded. We could enumerate loaded modules and look for the address of our window hook, but I didn't want to overcomplicate the initial implementation.

The commit message has a bunch of detail that I'll lazily paste below:


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.

The current implementation requires that the SDL code be running in a DLL
called SDL2.dll (the default SDL DLL name). If SDL2.dll is not loaded, the
hook will not be enabled.

[0]: https://docs.microsoft.com/en-us/windows/win32/dxtecharts/disabling-shortcut-keys-in-games