vk/renderer,hikaru/renderer: move core stuff to vk.
hikaru/renderer: fixed BACKGROUND polytype.

Used for e.g. the sky in PHARRIER.

It was a blending mode thing. I suspect the other polytypes may have similar
semantics.
hikaru/gpu: make get_texhead_str() output easier to parse.
hikaru/renderer: fix failed-to-decode-texture case.
hikaru/{gpu,renderer}: ALPHA8 are actually LUMINANCE8 + ALPHA8.

Fixes the clouds in PHARRIER.
hikaru/renderer: disable fogging by default.