* Re-implement PaintCallbacks With Support for WGPU
This makes breaking changes to the PaintCallback system, but makes it
flexible enough to support both the WGPU and glow backends with custom
rendering.
Also adds a WGPU equivalent to the glow demo for custom painting.
Based on https://github.com/hasenbanck/egui_wgpu_backend
`egui-wgpu` is now an official backend for `eframe` (opt-in).
Use the `wgpu` feature flag on `eframe` and the `NativeOptions::renderer` settings to pick it.
Co-authored-by: Nils Hasenbanck <nils@hasenbanck.de>
Co-authored-by: Sven Niederberger <niederberger@embotech.com>
Co-authored-by: Sven Niederberger <73159570+s-nie@users.noreply.github.com>
