Fixed a few issues bought up in code review.
VideoSoftware: Added support for RGB565_Z16 color format.

Multisampling and Z Compression aren't supported yet.

Because Z Compression isn't supported, it just falls back to a 24 bit z
buffer when formats other than ZC_LINEAR are selected.

Peach's Castle tries to use ZC_LINEAR (possibly an issue with the
default value of the zcontrol register) and has huge z fighting issues
when you manually enable AA in the start button menu.