WIP: Add GLSL_EXT_structured_descriptor_heap

[Tobski]

This extension is a work in progress, provided for comment.

#298 provides a basic interface for interacting with descriptor heaps; this extension is intended to enable applications to interact with descriptor heaps in a similar way to uniform buffers, such that descriptor data and POD can be mixed freely within the heaps.

[Tobski]

Note: This is based on the branch in #298. Assuming that's going to be merged soon anyway, I haven't rebased this.

[spencer-lunarg]

ping - is there anyone assigned to this, we are having massive issues in validation due to the current GLSL limitations

[Tobski]

@spencer-lunarg Not necessarily right now, it's something we kind of wanted to have sit for a bit and see if anyone gave feedback on. But I mean LunarG giving feedback that you need it does in fact count so... we might be able to start moving it ahead again.

Can you elaborate on what it is you want it for?

[spencer-lunarg]

Can you elaborate on what it is you want it for?

The current GLSL to use untyped pointer is not a joy to use

We just ran into yet-another issue yesterday because we are trying to hack everything around alignments from being at the heap starting address... (so basically the lack of heap_offset is hard to use untyped pointers currently for anything meaningful outside small tests)

[Tobski]

@spencer-lunarg I think the first question is - is there anything in the spec that is unclear, or you would like to see work slightly differently, or that you think is missing? I need to do a pass over the spec before anyone starts implementing this again. If you could do a review pass of some kind to highlight any issues, I can go fix those, then we can see if we can find someone to implement it.

[spencer-lunarg]

@Tobski went through it again, this extension spec looks good to me... the lack of this extension being available today is my "real issue"