This PR adds the ability to provide per-descriptor-set IO mapping shift
values.  If a particular binding does not land into a per-set value,
then it falls back to the prior behavior (global shifts per resource class).

Because there were already 6 copies of many different methods and internal
variables and functions, and this PR would have added 6 more, a new API is
introduced to cut down on replication and present a cleaner interface.
For the global (non-set-specific) API, the old entry points still exist
for backward compatibility, but are phrased internally in terms of the
following.

    // Resource type for IO resolver
    enum TResourceType {
        EResSampler,
        EResTexture,
        EResImage,
        EResUbo,
        EResSsbo,
        EResUav,
        EResCount
    };

Methods on TShader:

    void setShiftBinding(TResourceType res, unsigned int base);
    void setShiftBindingForSet(TResourceType res, unsigned int set, unsigned int base);

The first method replaces the 6 prior entry points of various spellings, which
exist now in depreciated form.  The second provides per-resource-set functionality.
Both accept an enum from the list above.

From the command line, the existing options can accept either a single shift value as
before, or a series of 1 or more [set offset] pairs.  Both can be provided, as in:

    ... --stb 20 --stb 2 25 3 30 ...

which will use the offset 20 for anything except descriptor set 2 (which uses 25) and
3 (which uses 30).