GPE engine - Standardize variables via snake_case

[nhurde]

Originally when engine began snake_case, camelCase and MACRO_CASE was used to show different levels of controls from local, temp and global variables.

Camel casing should be removed and replaced with snake_case and MACRO_CASE can be kept for globals and DEFINES / macros.

[nhurde]

Bringing to Summer 2024, as is important, but this is mainly refactoring and renaming variables/functions so functionality shouldn't be changed.