Improvements to Yamaha MU50 internal layout

[felipesanches]

Internal layout in this PR:

And with external layout (available at https://github.com/ArqueologiaDigital/MAME_synths_artwork/tree/main/mu50):

[cuavas]

I really don’t want to go in the direction of having SVG data that isn’t readily human-readable in internal layouts. So far, I’ve been trying to limit it to relatively simple shapes, coordinates that are simple to understand at a glance (nothing that needs a large number of significant figures), no unnecessary CSS, and no unnecessary transforms.

The idea of supporting embedded SVG in internal layouts wasn’t to have them bloated with stuff that really belongs in external artwork – it was just to allow people to add geometric shapes beyond circles and rectangles.