SDL Net API namespace consistency

[sbhutch]

The SDL and SDL image APIs capitalize the namespace prefix:

SDL_Init
IMG_Init

The SDL mixer and SDL net APIs do not:

Mix_Init
Net_Init

Unless this was deliberate, the transition to SDL3 would be a great time to make these APIs consistent.

[Sam Lantinga]

I don't think we're going to change that at this point. Thanks for the feedback!