Now that copy_deref are passed alignment information we should take it
into account when splitting var copies. Note that we always pass an
explicit alignment even if the copy is naturally aligned (which is
normally encoded with a 0), but that shouldn't be a problem.

We assume GL or CL size/align for natural alignment calculation, but
this can easily be extended through the addition of a new helper taking
a glsl_type_size_align_func.

The last thing that's worth mentioning is the fact that we get struct
field offsets with glsl_get_struct_field_offset() which returns the
cached offset value. That means we rely on proper offset calculation at
type creation. Another option would be to make
struct_type_get_field_offset() public which can be passed a
glsl_type_size_align_func.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>