Set eframe to fit its content

**Is your feature request related to a problem? Please describe.**
Eframe uses a specific size on creation. I want to measure the size of the content and set the size accordingly.

**Describe the solution you'd like**
I've hacked up a solution which works for me on my application on my OS. I wish (at least some) features would be part of egui, so they could be tested for other systems and thus be more dependable.

**Describe alternatives you've considered**
Currently using a eframe means to set it to a guessed (hopefully reasonable) default size and let the user resize it.

**Additional context**
I haven't found an issue on this repo which would already deal with that.

I found some discussions:
- [Automatically set initial window size to fit contents
#2858](https://github.com/emilk/egui/discussions/2858) Shows a solution which works on MacOS using a window to calculate the inner size. It links to:
- [How to dynamically resize eframe window? #4329
](https://github.com/emilk/egui/discussions/4329) detailing how to change a eframe size after creation.

I'm running Linux (CachyOS) with Gnome 49.1 on Wayland. I need two update cycles to pre-render the GUI in a egui::window. Then one cycle to create a CenterPanel. Then a cycle to resize the CenterPanel:
- [Application with minimal example](https://github.com/PJaros/eframe-fit-content)
- [Application which uses it](https://github.com/PJaros/menu_pdf)

So I wish those two features:
- a way to calculate the inner size without a egui::window
- a way to resize a eframe with no need to use an additional update cycle

Start by reviewing eframe's sizing behavior alongside the linked discussions and minimal example, focusing on the egui::window and CenterPanel approaches described in the issue. Done means providing a dependable way to calculate content-driven inner size without an egui::window and resize eframe without an extra update cycle.

Written by the indexing model from the issue text.