meshType: do not qualify vertices as a data slice; see #19

By not qualifying vertices as a data slice we are given the ability
to append a mesh to an empty mesh (an empty mesh has no vertices)
which is convenient in a few cases.
Mesh: fix two bugs with Mesh.append; see #19

appendData was incorrectly returning reflect.Value when we wanted
it to return an interface, this causes a crash because callers expect
to derive the underlying type from the interface value.

fixIndices was sometimes not intialized (in the case of appending a
non-indexed mesh to another non-indexed mesh), which caused a nil
pointer panic.