[Request] Support rendering existing `usvg::Tree`

Support loading SVG images directly from `usvg::Tree` that is already in memory.

**Is your feature request related to a problem? Please describe.**
I cannot render SVG images in egui with an already existing `usvg::Tree`.

**Describe the solution you'd like**
Some additional method on the svg_loader in `egui_extras` named something like `from_tree()` that would accept a `usvg::Tree`. Not quite sure how this would work, as I am not familiar with the internals of the library. Would be nice to not have to specify a URI in this function as it already knows what datatype is incoming.

**Describe alternatives you've considered**
My current solution is to use `egui::widgets::Image::from_bytes()` and convert the tree to a bytes array using `tree.to_string().into_bytes()` which introduces several additional conversion steps.

**Additional context**
I am building a library that uses `usvg::Tree` internally to parse custom data out of SVGs. As I already have the SVG parsed into a `tree`, it would be ideal to pass that directly to rendering code without having to go through converting to string, then bytes, then reparsing the SVG.

This is partially related to #7797.

Start with the SVG loader in egui_extras and review the related discussion in issue #7797. Determine how an existing usvg::Tree could enter the rendering path without a URI or serialization step; done means an additional loader method can render the in-memory tree directly.

Written by the indexing model from the issue text.