Fix rounding of ImageButton

[chriscate]

Updated ImageButton to use rounding from the corresponding image via self.image.image_options().rounding and added a rounding() method to ImageButton.

Closes #3486 if we agree the addition of a rounding() method on the ImageButton itself, which the issue also mentions, can be discussed and handled separately.

This now fully closes #3486

[chriscate]

One thing I noticed with the current approach in this PR is that when using e.g., rounding(50.0), the border bumps up against the available space and the edge of the circle flattens out ever so slightly (see bottom circle in below screenshot). Is there a way to auto-scale this to account for that?

I thought this existing logic would allow it all to fit:

egui/crates/egui/src/widgets/button.rs

Line 647 in f218825

.align_size_within_rect(image_size, rect.shrink2(padding));

Also, any insight into why rounding was called out separately from all the other image options to begin with? There must have been a reason.

[v-kat]

I'm fine with this although I'd also like to see the rounding() method. I have no clue why the original code separated out rounding....

[chriscate]

I actually agree we need the rounding() method at the same time or else there's no quick way to override if the user chooses. I went ahead and added it in this same PR.

So, now this fully closes #3486

@v-kat let me know what you think and then I'll open for review.

[v-kat]

I haven't tried it but it looks okay

[emilk]

Nice one, thanks!

[chriscate]

You're welcome! Happy to contribute.