preview: PAL60 mode in display.c is disabled

[thekovic]

At some point, the code to handle PAL60 in resolution_t.pal60 was disabled with an #if 0. I don't remember why this happened (if there was a bug or more research or design work needed) so I'm making this issue to keep track of it.

https://github.com/DragonMinded/libdragon/blob/preview/src/display.c#L364-L378

[rasky]

Given that it has little compatibility with framegrabbers, I think it should be removed as an option from resolution_t, and added as a custom preset to vi.c, exposing an API to change the preset (which is currently hardcoded depending on the TV type).

[thekovic]

Fixed by #805 , I believe