Clean up generatemip.h. * 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/202503 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>