glsl: buffer variables cannot be defined outside interface blocks

glsl: buffer variables cannot be defined outside interface blocks

Section 4.3.7 "Buffer Variables", GLSL 4.30 spec:

"Buffer variables may only be declared inside interface blocks
(section 4.3.9 “Interface Blocks”), which are then referred to as
shader storage blocks. It is a compile-time error to declare buffer
variables at global scope (outside a block)."

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>