core: WritePixel: Print Unimplemented on 18BPP

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
core,gfx: Fix HLine & WritePixels.

* Offset in write pixels is now passed in bits

* The GP_HLine uses correct macros for computing
  offsets
  (Fixes modulo by zero on RGB666)

Signed-off-by: Cyril Hrubis <metan@ucw.cz>