egui-wgpu: Add a wgpu texture

[parasyte]

Is your feature request related to a problem? Please describe.

egui-wgpu has a method to upload raw image pixels and create an egui texture. It is missing a method to allocate an egui texture from a wgpu texture. This was available in egui_wgpu_backend: https://docs.rs/egui_wgpu_backend/0.17.0/egui_wgpu_backend/struct.RenderPass.html#method.egui_texture_from_wgpu_texture

Describe the solution you'd like

Add a similar method to egui-wgpu so that any texture usable by wgpu can also be used directly in egui.

Describe alternatives you've considered

• Stick with egui_wgpu_backend instead and update it to be compatible with egui 0.18. This was originally tracked in Bump to latest egui (0.18.1 as of time of writing) hasenbanck/egui_wgpu_backend#60, but closed when egui-wgpu became a viable alternative.
• Animated textures can be updated every frame with the existing update_texture method, but this is unnecessary and probably a performance hit.

Additional context

This is an important feature for pixels, which provides access to the wgpu texture view. And generally important for any application that renders to a texture.

[emilk]

closed in #1660