Add touchpad magnify gesture support for Windows.

[killianc3]

After seeing touchpad gestures implemented on macOS ( #2157 ), I looked for an implementation for Windows.

According to the microsfot documentation, there is a WM_GESTURE message that can be processed to support touchpad gestures. But at first I couldn't get one of these messages, then after searching, it seems that no one has implemented this message yet.

Then, in the microsoft doc, I saw this table https://bit.ly/3LDVdRd which describes how the gestures correspond to the existing messages, so based on that and on #2157, I implemented the zoom gesture.

What do you think about it ?
Thanks in advance.

[killianc3]

I was thinking that maybe some people have already implemented the magnification gesture in their code by checking if the control key is activated when they receive a MouseWheel event. But with these changes, their code might not work on Windows anymore because MouseWheel will no longer be emitted.

And I also follow this #2157 wich is also a note as a break change.

[madsmtm]

Yeah, I could guess - I was more thinking explaining it in the changelog, so that the user knows "oh, I gotta check for my usage of MouseWheel, and see if I do that"

[killianc3]

Oh yes, I see, so something like that should be more understandable ?

[madsmtm]

Yup, that looks wonderful.

Can't say anything about the implementation, @msiglreith will likely do that at some point

[msiglreith]

The implementation looks fine but I'm a bit unsure about doing this inside winit:

• There seems no clear mapping from mouse wheel values to magnification factor. Mapping from -1/1 to zoom factors seems more application specific. Support Direct Manipulation or other Windows API in the future might lead to inconsistent behavior here.
• Not totally sure about other platforms but such a keybind should be platform-agnostic.

[killianc3]

Yes, you're right, after thinking about it a bit, I also think these changes are not very much in line with Winit's goals, my bad.

Thanks for your time !