Improve case consistency for structs

[jgcodes2020]

Here's a few structs from the current m64p_types.h.

https://github.com/mupen64plus/mupen64plus-core/blob/860fac3fbae94194a392c1d9857e185eda6d083e/src/api/m64p_types.h#L255-L273 https://github.com/mupen64plus/mupen64plus-core/blob/860fac3fbae94194a392c1d9857e185eda6d083e/src/api/m64p_types.h#L275-L290 https://github.com/mupen64plus/mupen64plus-core/blob/860fac3fbae94194a392c1d9857e185eda6d083e/src/api/m64p_types.h#L182-L221

It would be a good code-formatting thing to stick with a common casing convention for these structs. Doing so will also make it easier to create bindings for other languages.