Implement pen and touch support for Linux

[cgutman]

Description

The first commit resolves a longstanding issue with how Linux input was treating absolute mouse input. Rather than emulating an absolute pointing device, Sunshine was actually emulating a single-touch touchscreen. This lead to all sorts of strange things depending on how the desktop environment treats touch input. Most commonly, it caused the cursor to disappear when not moving or mouse clicks to go to the wrong window. Now we behave like the Linux vmmouse driver with separate absolute and relative mouse devices.

The second commit implements native support for pen and touch on Linux. I've tested with a few apps under a GNOME Wayland session plus evtest and the events look sane to me.

Testing checklist:

• GNOME Wayland
• GNOME X11
• KDE Wayland
• Sway

Screenshot

Issues Fixed or Closed

Fixes #1828
Closes #1706 (superseded)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated