Workaround libpng crash on macos by not creating NSImage from png data

[Wumpf]

• Fixes Basic egui stripped down example breaks #7155

On some Macs (OS version dependent?) NSImage may load user installed libpng and subsequently crash (likely if they don't expected symbols). This PR works around this by not encoding back to png but instead sending the image over directly. This is strictly speaking better anyways since we don't have to re-encode icons, but is also a bit more cumbersome to do

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7252-fix-icon-setting-crash-maybe
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

Should we decide to merge this, we have to remember to also handle the default icon differently (by including it as a raw image or using the image crate to decode it?)

[Wumpf]

If I'm not reading this wrong we so far decoded the default icon and then encoded it again...
I'd rather keep the default icon encoded just out of size consideration. Even better though would be to generate it from code: I mean it's a lil rounded rectangle with an 'e' in it 😆
Counter point: this could mean that we don't have to depend on the png feature of image 💡

[Wumpf]

Fix got confirmed! 🥳

[lucasmerlin]

Not related to your PR, but I noticed that the icon will switch to the terminal icon for a split second when closing the app 🤔

Untitled.mov

Not a huge problem but it's a bit odd

[lucasmerlin]

Nice!