More PackedVector Fixes

I fixed a bunch of bugs in the PackedVector types adding in unit tests as I went:

 - NormalizedShort2/4 both had incorrect packing and unpacking.
 - Bgra4444 didn't clamp input values.
 - Bgra4444 wasn't swizzling to BGRA.
 - Short4 wasn't packing correctly.
 - Bgr565 wasn't doing input clamping.

These changes match XNA behavior 100%.