Add prefix byte for extended scancodes on Windows

[vickles]

Currently on Windows only the low byte of scancodes is passed back through the KeyboardInput{ scancode } field. This causes scancode collisions between different keys.

Some examples that I've run up against in my game project:

• Both HOME and NumPad 7 map to 0x47. HOME should be 0xE0 47, and NumPad 7 should be 0x47.
• Both UP and NumPad 8 map to 0x48. UP should be 0xE0 48, and NumPad 8 should be 0x48.

I've been referencing this Microsoft doc: Keyboard Scan Code Specification. It seems to contains the full breakdown of how scancodes are defined on windows.

This change prepends the prefix byte to scancodes which have the extended bit set. It allows me to distinguish between keys properly by scancode, and aligns them with the codes in that spec doc.

[kchibisov]

@vickles just fyi, since you rebase that patch from time to time, we haven't forgot about it, but since neither @chrisduerr nor me maintaining Windows, we're not that fast to take actions when it comes to it. However if it'll take too long for Windows maintainer(s) to appear we'll likely go back to it ourselves, even though we don't use Windows in particular.

[vickles]

No problem. I'm updating my branch both to make sure it stays mergeable here, and also because I'm using that branch on my fork as the source of winit in my project

[msiglreith]

This affects the PAUSE/SCROLL logic below as it doesn't filter the additional event caused by RAWINPUT:

PR:

DeviceEvent { device_id: DeviceId(DeviceId(65599)), event: Key(KeyboardInput { scancode: 57373, state: Released, virtual_keycode: Some(Pause), modifiers: (empty) }) }
WindowEvent { window_id: WindowId(WindowId(0x75065e)), event: KeyboardInput { device_id: DeviceId(DeviceId(0)), input: KeyboardInput { scancode: 69, state: Released, virtual_keycode: Some(Pause), modifiers: (empty) }, is_synthetic: false } }
DeviceEvent { device_id: DeviceId(DeviceId(65599)), event: Key(KeyboardInput { scancode: 69, state: Released, virtual_keycode: Some(Pause), modifiers: (empty) }) }

master branch:

WindowEvent { window_id: WindowId(WindowId(0xd06cc)), event: KeyboardInput { device_id:  DeviceId(DeviceId(0)), input: KeyboardInput { scancode: 69, state: Released, virtual_keycode: Some(Pause), modifiers: (empty) }, is_synthetic: false } }
DeviceEvent { device_id: DeviceId(DeviceId(65599)), event: Key(KeyboardInput { scancode: 69, state: Released, virtual_keycode: Some(Pause), modifiers: (empty) }) }

[vickles]

I've updated this logic. I tested master and found it was reporting the same scancode and virtual key for pause and numlock. The updated version should handle them both as separate entities better. I also tested scroll lock, pause and numlock with a variety of modifiers and it seems to be looking happy now.