#35 Port Tux Paint to SDL3

[William Kendrick]

SDL3 offers some new features that will greatly benefit Tux Paint, including:

• SDL_Pen - an API for pressure-sensitive pen (stylus and/or eraser) handling, e.g., for input and drawing tablets or suitably equipped mobile / tablet devices
• SDL_Camera - [an API that] lets apps read input from video sources, like webcams
• SDL_GetSystemTheme - query the preferred desktop theme (light or dark)
• Filesystem and file dialog functionality
• GPU API - a cross-platform way for apps to talk to modern graphics hardware [which] offers both 3D graphics and compute support

A migration guide (SDL 2.0 to 3.0) is available. (They even provide a set of scripts to rename things (function calls, headers, etc.), which might be a good "Step 1" of this task.)

[Pere Pujal i Carabantes]

Hi Bill,
I have something that begins to work, it is far from finished thought.
Would be OK to push it into a new SDL3 branch?