Rework WindowMode::Fullscreen API

[jf908]

Objective

• Provide a mechanism for selecting a VideoMode for FullScreen Exclusive #16883
• Improve the default behaviour of the exclusive fullscreen API.

Solution

This PR changes the exclusive fullscreen window mode to require the type WindowMode::Fullscreen(MonitorSelection, VideoModeSelection) and removes WindowMode::SizedFullscreen. This API somewhat intentionally more closely resembles Winit 0.31's upcoming fullscreen and video mode API.

The new VideoModeSelection enum is specified as follows:

pub enum VideoModeSelection {
    /// Uses the video mode that the monitor is already in.
    Current,
    /// Uses a given [`crate::monitor::VideoMode`]. A list of video modes supported by the monitor
    /// is supplied by [`crate::monitor::Monitor::video_modes`].
    Specific(VideoMode),
}

Changing default behaviour

This might be contentious because it removes the previous behaviour of WindowMode::Fullscreen which selected the highest resolution possible. While the previous behaviour would be quite easy to re-implement as additional options, or as an impl method on Monitor, I would argue that this isn't an implementation that should be encouraged.

From the perspective of a Windows user, I prefer what the majority of modern games do when entering fullscreen which is to preserve the OS's current resolution settings, which allows exclusive fullscreen to be entered faster, and to only have it change if I manually select it in either the options of the game or the OS. The highest resolution available is not necessarily what the user prefers.

I am open to changing this if I have just missed a good use case for it.

Likewise, the only functionality that WindowMode::SizedFullscreen provided was that it selected the resolution closest to the current size of the window so it was removed since this behaviour can be replicated via the new VideoModeSelection::Specific if necessary.

Testing

• Tested only on Windows 11 so far.
• macOS/Linux need to be tested.

Migration Guide

WindowMode::SizedFullscreen(MonitorSelection) and WindowMode::Fullscreen(MonitorSelection) has become WindowMode::Fullscreen(MonitorSelection, VideoModeSelection). Previously, the VideoMode was selected based on the closest resolution to the current window size for SizedFullscreen and the largest resolution for Fullscreen. It is possible to replicate that behaviour by searching Monitor::video_modes and selecting it with VideoModeSelection::Specific(VideoMode) but it is recommended to use VideoModeSelection::Current as the default video mode when entering fullscreen.