Try to optimize inline matrix loads.

Improves performance by a few percent in Gods Eater Burst.
Avoid flushing if the bone matrix is the same.
Optimize loading of texgen matrices.

Pretty small impact, may help games that use them a lot.
Try to load view and model matrices a bit faster.
Merge pull request #5603 from unknownbrackets/gpu-minor

Try to load inline matrices a bit faster
Oops, forgot the world matrix too.

VerySleepy is telling me that time is spent in WORLDMATRIXDATA in games,
but I didn't check the perf impact exactly.  It's probably small, but may
help some games.
GCC buildfix
Fix a typo.
Properly reset some state between games.
Consistently use MIPS_REG_V0 for return values.

This way all instances are a lot easier to search for (was trying to find
where we were writing an incorrect value to v0, turns out to be callback
related of course.)
Merge remote-tracking branch 'upstream/master' into fps2