Increase .mscz thumbnail size

[benwaffle]

As discussed in #27136 (comment), I'm doubling the default thumbnail size which will be used in the Home tab, and the future macOS thumbnailer.

Before	After

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[Jojo-Schmitz]

Is this PR also correction the height/width ratio?
A score in e.g. A5 landscape currently shows as a stretched portait in the Home tab

[benwaffle]

No, it doesn't change the ratio

[Jojo-Schmitz]

Guess I'd need to open an issue for this...

[cbjeukendrup]

#27614