* materials: introduce MaterialCache
Presently, Filament Materials are instantiated by first parsing a bunch of
read-only data from a material file, then applying a bunch of options from its
Builder before settling on a final, immutable Material object. If two different
Material instances need to be parameterized differently, e.g. setting their spec
constants independently, each has to do all of these steps independently for
each variation.
This change introduces two new concepts: MaterialDefinition, representing the
deserialized, read-only state of a material file, and MaterialCache, a
reference-counted system responsible for managing the lifetimes of
MaterialDefinitions. Now, each Material asks the cache if a MaterialDefinition
exists for the particular UUID of the data it's trying to read; if not,
MaterialCache creates a new entry transparently. If a hundred different
Materials all try to load the same material data, only one
MaterialDefinition (and its associated GPU resources) will be created.
This first PR is the least possible invasive implementation of this feature.
There are a lot of room for improvements (and more planned). For example, each
Material still manages its own compiled shader program cache, but we can easily
move this to the MaterialCache in future PRs, further enabling the planned
mutable spec constants feature.
Additionally, there's room here to add a Material::toBuilder() method, which
could take an extant material and create a Builder object from it already
parameterized with all of the same options, a la the prototype design pattern.
* material cache: key on crc32
* material cache: make materialParser private
* move RefCountedMap to utils and add unit tests
* material cache: make create functions private
* material cache: fix broken tests on iOS/web
* material cache: address more comments
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