hikaru/gpu: ignore conditional recall viewport instructions.

Now SGNASCAR looks a *lot* better.
hikaru/gpu: annotate (and store) alpha test flag.

And improve annotations for related instructions.
hikaru/gpu: store thresholds for alpha test.
hikaru/gpu: ignore conditional version of instruction 161.

Used in several places.
hikaru/gpu: remove annoying check.

It was way more annoying than useful.
hikaru/gpu: add a note about a serious issue with the IDMA.
hikaru/{gpu,viewer}: simplify register layout in gpu_t.
hikaru/gpu: remove frame_type.

It was not synchronized to the frame type. Doh.
hikaru/gpu: simplify cp layout in gpu_t.

Also: add a macro to access the SP (it's shorter, yeah!) and move the
PC, SP, and UNHANDLED macro definitions to gpu-private.h, so they can be
reused elsewhere.
hikaru/{gpu,renderer}: group all 2D and 3D state within gpu_t.

And rationalize the names a bit, reflecting "our" current and improved
understanding of the 3D hardware.

Also properly clear the state in on_cp_begin ().
cpu/sh4, hikaru: group registers together in sh4_t.

And consequently simpligy reset() and {save,load}_state().

Fix fallback in hikaru.c.
hikar/renderer: don't memset the whole 'mesh' structure.

It is huge! As a result, the frame rate magically doubled.
hikaru/gpu: implement viewport stack.

Information kindly contributed by DreamZzz.
hikaru/gpu: temporarily disable mesh instancing.

Until it is better understood.