Start the Hcount at the beginning of vblank.
Implement sceDisplayAdjustAccumulatedHcount().

And also make sure sceDisplayGetAccumulatedHcount() works right.
Spend a lot less time calculating the hcount.

Tiny loss in precision, can't even notice.  Cuts time significantly in
Valkyrie Profile.
Merge pull request #3587 from unknownbrackets/display2

Calculate Hcount more accurately, support adjusting