Make {ttf, image, mixer, gfx}::ffi public

[Meralis40]

This PR is the first part for resolve #647.

It:

• merge all private gfx::{framerate, imagefilter, primitives, rotozoom}::ll into gfx::ffi
• mark {ttf, image, mixer, gfx}::ffi pub

[Cobrand]

I feel like if we're going to do it, we're better off doing it all at once. If you're not up for moving evertyhing ffi into sdl2-sys, I'll take care of it. Please tell me if you're up for if or not.

[Cobrand]

Unless I'm mistaken, since #782 everything is available from sdl2-sys directly, so I'm closing this. Still, thank you for the PR!