layers:Add BINDABLE superclass for image/buffer state

layers:Add BINDABLE superclass for image/buffer state

Support for sparse memory binding is virually non-existant in current
validation layers. This is a first step down a long path to improve
sparse binding support.

The BINDABLE superclass is sub-classed by IMAGE & BUFFER state objs
and contains memory binding info in both the single binding and
sparse binding cases.

Bindings are initially encapsulated in the new MEM_BINDING struct.
For single binding cases, things are almost exactly the same as they
have always been. For sparse binding, the various bindings will be
stored in the sparse_bindings set where we initally don't have complete
information on the sparse binding as we only store the MEM_BINDING info
but this is a start.