Use less RAM: remove egui images from ImageStore

[Wumpf]

This requires rendering the hover preview also with re_renderer

Safes us a ton of overhead and memory for everything with images!

[emilk]

We could still use egui::Image if we register the texture id:s with egui_wgpu::Renderer (I know people do such things with the egui_glow backend). Not sure what is easiest, but worth inestigating imho.

[emilk]

We could also consider never using egui::Image and instead use a 2D Space View when displaying images in the UI (or some other use of re_renderer). The advantage of that would be that the rendering of images would be more consistent, which will be important once we start supporting images outside of sRGB, and with increased bit depth, etc.

[teh-cmc]

Fixed during recent colormapping work.