Output register components may not be written more than once

[neobrain]

According to fincs on efnet/#3dsdev, August 14, 2015:

18:39 < fincs> You can't write to an output register more than once
18:40 < fincs> Last time I checked it crashed, but that was before the libctru GPU freezing bug (TM) was fixed

This should be verified, and nihstro should error out when trying to compile shaders which don't write all output registers.

Along with #24, this means nihstro should guarantee that all declared output register components are written exactly once.