Provide Skia as opt-in renderer

[tronical]

This issue tracks the progress of allowing users to select a Skia based renderer for Slint.

After the initial POC this now involves a few steps:

• [x] #1447
• [x] Merge the initial branch that compiles with GL support on macOS, Linux and Windows (no cross-builds yet): #1449
• [x] #1454
• [x] #1448
• [ ] Add support for text input (selection and cursor)
• [x] #1470 (optional)
• [ ] #1471 (optional)
• [ ] #1451
• [ ] Skia: Add support for using Vulkan (optional)
• [ ] Add documentation that explains how to opt into the renderer at compile and build time for Rust and C++ (including Yocto related bits)

One merged, the front-end crates/CMake interface will have a renderer-skia feature, next to the existing renderer-femtovg feature.