Avoid raw reinterpret_casts in the FBX parser

As reinterpret_cast can break strict aliasing rules, causing runtime failure
on Android, replace such usage in FBXParser with memcpy.  Also provide a
utility routine for both performing the copy and asserting the validity of
the buffer length relative to the copied region.
Merge pull request #334 from jdduke/fbx_binary_fix

Avoid raw reinterpret_casts in the FBX parser