refactor: move gfx::ImageSkia functions to their own util file

[erickzhao]

Description of Change

Inside atom_api_native_image.cc, we have a set of functions that are useful for generating gfx::ImageSkia representations.

I wanted to use those outside the context of NativeWindow in #19420, where I am reusing electron::ShowMessageBoxSync function to pop up an About dialog for Windows.

For this About Dialog, I am grabbing the message box icon from a file path, like we do in Linux.

Since gfx::ImageSkia is used outside of NativeImage, I was thinking that it'd be useful to do extract these utility functions into their own shared file instead of keeping them private within the NativeImage API file.

Would like feedback on the usefulness/viability of this!

cc @codebytere @deermichel

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines

Release Notes

Notes: no-notes

[codebytere]

this is just a refactor with no function change of any kind so this seems fine!

[nornagon]

seems fine to me.

[release-clerk]

No Release Notes