Calculate Hcount more accurately, support adjusting

This is an oldie but a goodie.  90b324a4b0bff215e5f2571dd8b9acc90208abb1 is from March, but I never bothered myself to test it.  Since Star Ocean was calling the one function I thought it'd be worth it finally.

Notes:
 * I think someone once said a game got slower when this was moved, which is why I never worried about it.  But it always seemed right for the timing to start there.
 * I think the test in hrydgard/pspautotests#99 shows that the behavior is correct, since it starts at 1 at the beginning of the vblank.
 * The wrap around is probably important.  Test should show this behavior too.  **I think it fixed Star Ocean.**
 * Also implements sceDisplayGetAccumulatedHcount() which reporting shows some games calling.
 * We still don't match exactly.  On my device, hcount goes up to 14 while inside vblank, but PPSSPP only 13.  Probably not major.  Our average is also 285.

I spent a little too long forgetting that floats don't wrap around when you add to them, so I don't have a lot of time to test.  However, I just did ~20 battles or so and can no longer reproduce the issue (where I could every other or so before.)

Have not tested too many other games unfortunately, but I checked for perf regressions in a couple that call these funcs.

-[Unknown]