Blend shapes did not take into account octahedral compressed vertex
attribute layouts and this resulted in incorrect lighting on the
resulting blended mesh
Now make the blend_shapes shader octahedral compression aware!
(cherry picked from commit 50ed674ec2)
As of clang-format 6.0.1, putting the `/* clang-format off */` hint
around our "invalid" `[vertex]` and `[shader]` statements isn't enough
to prevent a bogus indent of the next comments and first valid statement,
so we need to enclose that first valid statement in the unformatted chunk.
