glsl: Add GLSL_TYPE_INTERFACE

glsl: Add GLSL_TYPE_INTERFACE

Interfaces are structurally identical to structures from the compiler's
point of view.  They have some additional restrictions, and generally
GPUs use different instructions to access them.  Using a different base
type should make this a bit easier.

This commit also adds the glsl_type::interface_packing fields.  For
GLSL_TYPE_INTERFACE types, this will track the specified packing mode.
It is analogous to gl_uniform_buffer::_Packing.

v2: Add serveral missing GLSL_TYPE_INTERFACE cases in switch-statements.

v3: Add information about glsl_type::interface_packing.  Move row_major
checking in glsl_type::record_key_compare from this patch to the
previous patch.  Both suggested by Paul Berry.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>