glsl: add subpass image type (v2)

glsl: add subpass image type (v2)

SPIR-V/Vulkan have a special image type for input attachments
called the subpass type. It has different characteristics than
other images types.

The main one being it can only be an input image to fragment
shaders and loads from it are relative to the frag coord.

This adds support for it to the GLSL types. Unfortunately
we've run out of space in the sampler dim in types, so we
need to use another bit.

v2: Fixup subpass input name (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>