Use UTF-8 encoding in shader viewer to fix character corruption

[qwmnerbvqwmn]

Description

Ensures the shader viewer uses UTF-8 encoding when loading source files, preventing character corruption (e.g., in comments) for non-ASCII characters.

The current UTF-8 text displays as unreadable characters, but displays normally after modification.

[baldurk]

Thanks for this change, it looks good and I honestly didn't realise scintilla didn't default to UTF-8 since that's what everyone should be using really.

As a general note for the future - please try to avoid creating so many PRs at once. It's better to create them as you make changes, to more evenly spread the review burden. We'll look over the PRs over the next while not all at once.