demosaic: more Markesteijn clean-up/optimization

- as per Ingo Weyrich, set TS to 88 to try to keep data in a
  reasonable sized L2 cache
- cleanup buffer sizes/offsetes a bit more
- optimization from Ingo Weyrich, make yuv indexed by channel first
- use sizeof(uint8_t) in memset, even if it is 1
- for a few uint8_t-derived local variable, type them as uint8_t