Add option to show a grid of banners

[BenHetherington]

This adds an option to show a 3x4 grid of 3:1 banners in the game selection menu, allowing game banners to be displayed without needing them to be stretched.

This can be configured using a menu_grid_layout .ini setting; set this to square_icons for the existing layout, and to banners for this new layout. The default is square_icons (the status quo).

[BenHetherington]

While tinkering with this, I had another idea for how this could be laid out – by scaling the banners by 0.75x, it's possible to fit a 4x4 grid on-screen (leaving more vertical space between rows), which might be preferable for those with a lot of games.

I'll add this as an extra commit, which can be used by setting menu_grid_layout to small_banners – let me know if you'd rather split this commit out into a separate PR!