* Move "private" functions into generatemip.inl.
* Use size_t for all dimensions.
* Use uint8_t pointers instead of unsigned char pointers.

BUG=angle:666

Change-Id: I8a1324802e4d670c042e0961c2b6f6fd53f4ebc7
Reviewed-on: https://chromium-review.googlesource.com/202503Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>