GlslangToSpv: Pass the pointer directly into the client function for opaque types

For opaque types such as samplers, images, and atomic counters, we want to
reference the actual object in the child function.  For a long time, we
used a shadow variable and made a copy of the image/sampler.  In 76d0ac1a,
this was changed to not shadow samplers.  However, this didn't cover all
opaque types and it also didn't get the pointer storage classes right.
This commit fixes both of these issues.

Fixes #324