Added HalfVector4 and Short4
Fixes issue #1133.
Added the HalfVector4 and Short4 types.  Still missing several PackedVector types.
All projects now consistently include all currently implemented PackedVector types.
Merge branch 'develop3d' of https://github.com/mono/MonoGame into packed-vector
Merge pull request #1139 from slygamer/packed-vector

Added HalfVector4 and Short4