overridable g1 texture filters

[sh-dave]

Is g1 expected to be used with render targets, or is it more of an example and you expect users to do the rendering into the framebuffer manually?

With this PR it's possible to override the texture filter settings via extern declarations.

extern kinc_g4_texture_filter_t kinc_internal_g1_texture_minf;
extern kinc_g4_texture_filter_t kinc_internal_g1_texture_magf;

int main(...) {
...
  kinc_internal_g1_texture_minf = KINC_G4_TEXTURE_FILTER_POINT;
  kinc_internal_g1_texture_magf = KINC_G4_TEXTURE_FILTER_POINT;
...
}

I'm currently just playing a bit with the render targets, so feel free to just close this if it doesn't fit.

[RobDangerous]

"internal" means please only touch this internally so that doesn't quite fit. Making it fit for render-target use would be nice though.

[sh-dave]

Ok, added some setter functions. Not sure if the separate enum is overkill, but it felt kinda weird to include and use G4 types in the public api.

[RobDangerous]

It's good like that with those enums. And it's going in, thanks!