[Feature Request] Support trackpad as touch in Windows and Linux, matching `SDL_HINT_TRACKPAD_IS_TOUCH_ONLY` behavior

[chenx-dust]

Intro

Currently, most laptops has a multi-touch supported trackpad, but SDL cannot treat them as touch device except macOS. While SDL_HINT_TRACKPAD_IS_TOUCH_ONLY has been implemented for so long time in macOS, this behavior is still missing on other major desktop platforms.

Potential Methods

• Windows: Windows HID API can listen raw report from Precision Touchpad.
• Linux: libinput or evdev can listen as well.

Expected Behavior

When the hint SDL_HINT_TRACKPAD_IS_TOUCH_ONLY is enabled, trackpads should be registered and treated as touch devices on Windows and Linux, just like they currently are on macOS.

It may be even better if trackpad can be divided into a separate category for better handling it.

[Kontrabant]

Linux: libinput or evdev can listen as well.

libinput doesn't have an API for passing through raw touch events from trackpads, and, while you could open an evdev device to grab raw touch data, by default evdev devices for pointers and keyboards require elevated permissions to open, which would greatly limit the usefulness of such a feature, since people tend not to run random apps as root. It's also not necessarily always possible to correlate a raw evdev device to an abstracted higher level pointer device as exposed by X and Wayland, so the touches wouldn't necessarily be correlated with the proper device when multiple mice/trackpads are present.

Is there an actual use case for this? SDL currently passes through pinch gestures, and others, such as swipes, could be added.

[chenx-dust]

I'm actually working on a fork of moonlight (qiin2333/moonlight-qt#116), a streaming client, making touch event pass through to the host. This could deliver much better trackpad experience for streaming and remote desktop client, just like a native one.

Apart from that, there are some other use cases, such as using the trackpad to sign, play rhythm games, and so on. This feature is indeed not a very common need. If it can be realized, I would be very grateful. :)