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>

src/mesa/program/ir_to_mesa.cpp

@@ -625,6 +625,7 @@ type_size(const struct glsl_type *type)
       return 1;
    case GLSL_TYPE_VOID:
    case GLSL_TYPE_ERROR:
+   case GLSL_TYPE_INTERFACE:
       assert(!"Invalid type in type_size");
       break;
    }
@@ -2536,6 +2537,7 @@ _mesa_associate_uniform_storage(struct gl_context *ctx,
          case GLSL_TYPE_VOID:
          case GLSL_TYPE_STRUCT:
          case GLSL_TYPE_ERROR:
+         case GLSL_TYPE_INTERFACE:
 	    assert(!"Should not get here.");
 	    break;
 	 }