We add support for this by using global scratch values as storage for the structs in
uniform blocks. Any structs in std140 layouts that are referenced by value are initialized
in the shader scope, without any packing, so the type of the structs are equivalent with what
a GLSL program would expect.

TRAC #23327

Signed-off-by: Geoff Lang
Signed-off-by: Shannon Woods
Authored-by: Jamie Madill