Add transparency to vibraphone icon

[softins]

Short description of changes

Updates the vibraphone icon (introduced in #2080) to include a transparent background.

Context: Fixes an issue?

With the original vibraphone icon, the channel label looks like this:

With the replacement icon, it looks like this:

All the other instrument icons already have a transparent background.

Does this change need documentation? What needs to be documented and how?

No documentation changes.

Status of this Pull Request

Tested and working.

What is missing until this pull request can be merged?

Ready to merge.

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[pljones]

Can you show it in Compact, too, compared with some of the other icons, preferably (e.g. guitar/vocals).

[softins]

Can you show it in Compact, too, compared with some of the other icons, preferably (e.g. guitar/vocals).

Sure, I haven't changed the icon size (except reduced the height from 28px to 27px to match all the others), just its transparency.

Here is the old icon in Fancy mode:

And Normal mode:

And Compact mode:

Now the new icon in Fancy mode:

And Normal mode:

And Compact mode:

[pljones]

Yup, looks better. So now icon reviews need:

• ideally all platforms
• all fader sizes
• all background shades (to avoid things like a pale green icon "vanishing"...)

[softins]

I don't think there's anything contentious here, so I'll just merge as admin with one approval.