* new feature level API for backends
backend can now return a "feature level", each level corresponds to a
"bundle" of features.
Level1: ES3.0 capabilities
Level2: ES3.1 capabilities + 31 textures + cubemap arrays
Currently metal always returns level 1, GL and Vulkan return level 2
if 31 textures or more are supported.
* Add public APIs for feature levels
* Add infrastructure to check feature levels in materials
* validate material feature level on use
The validation is done when creating a renderable. If the engine doesn't
support the material's feature level, an exception is thrown (or assert
if exceptions are not enabled).
* material documentation
* activate ESSL 3.10 for feature level 2
also generate #defines to identify available feature levels
* support for cubemap arrays in the public API
if feature level 2 is supported, cubemap arrays can be used from the
public API.
* add release notes
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