build: add example cross-compile script for Win64 on MinGW32
decouple input depth from internal depth; allow input depth != internal depth

Shift and mask input samples as necessary to reach internal depth. The input
depth is no longer in x265_param, where it never really belonged since
pic.bitDepth was the only number that mattered.

Add validations for input color space and bit depths. No CLI option was added
for internal bit depth since it is not currently runtime configurable.
enforce new color space requirements, improve help
uncrustify all source - mechanically apply coding style
non-x86 build fixes