Validator: Support VK_EXT_scalar_block_layout

Adds validator option to specify scalar block layout rules.

Both VK_KHR_relax_block_layout and VK_EXT_scalar_block_layout can be
enabled at the same time.  But scalar block layout is as permissive
as relax block layout.

Also, scalar block layout does not require padding at the end of a
struct.

Add test for scalar layout testing ArrayStride 12 on array of vec3s

Cleanup: The internal getSize method does not need a round-up argument,
so remove it.