Align vertex attributes to multiples of 4 bytes

[lilleyse]

For performance and compatibility reasons, each element of a vertex attribute must be aligned to 4-byte boundaries inside bufferView (i.e., accessor.byteOffset and bufferView.byteStride must be multiples of 4).

The 1.0 to 2.0 upgrade step is currently ignoring this note in the spec. The fix will likely involve adding padding to the buffer. More discussion in #421.