Add drop shadow to main menu icons

[wlhlm]

The icons were a bit hard to distinguish on certain backgrounds. This adds a drop shadow like we use on the menu button text labels.

The website and Discord icons were excessively large, so scale them down to 64px like the GitHub icon. Additionally, the Discord icon had a non-square aspect ratio causing it to be rendered slightly squashed.

Before:

After:

[boubou19]

much better, thanks for fixing this, when i did those, i had no idea on how to make them like this.