Add {get/set}_window_position for Windows platform.

[mandroll]

Add get_window_position and set_window_position to miniquad for Windows platform.

• set_window_position uses the typical Request code path.
• get_window_position adds a screen_position field to NativeDisplay struct and updates it every time the screen position changes or is resized in the main update loop.

Background

My motivation for this is to implement some form of pseudo hot-reloading (recompiling and reloading game state), which requires the window to always be in the same position and size. I've included an example in this gist, with a short video demonstration here.

There have also been numerous requests for this feature in the Discord channel and also #329.

Other platforms

This PR only adds Windows support. Other OSes will give an unimplemented error when trying to use these features. It is not applicable to ios/android/wasm platforms. For other platforms (linux_*, macos), adding support involves:

1. Add a set_window_position implementation and calling it in process_request
2. Update NativeDisplay.screen_position in the main update loop for that platform.

[not-fl3]

Thanks for PR!