* materials: introduce mutable spec constants
Rationale & design of this feature has been discussed internally.
The current implementation uses a `FixedCapacityVector` to store the new program
handles, but I wouldn't object to replacing it with a hasmap as discussed
offline.
I have compiled but not tested this yet on Android, so I'm not certain that the
API bindings are correctly wired up.
* materials: mutable spec constant feedback
* materials: address mutable spec constant comments
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