Fix compiler warnings for VS2010 /W4 in intra.c.

- Working towards issue #11.
- Removed intra_get_block_mode as unused.
- Removed unused parameters from functions. Many of them were remnants from
  earlier data structures and earlier features of HEVC that have been removed.
- Lots of implicit conversions from larger types to smaller ones. I tried to
  avoid turning all of them to explicit ones this time and opted for changing
  the original data type instead. Had to do it in few cases though to stop the
  changes from propagating too widely.