Rename the pixel conversion performance test

We want to have other tests related to pixel conversion, so avoid
possible naming conflicts.
Add missing copyright header.
PixelTransformer is gone so we can protect these now
Add pixel conversion correctness test
Fix endian conversion

This was just plain wrong, so redo it properly.
Merge branch 'convtest' of https://github.com/CendioOssman/tigervnc