new feature level API for backends (#5784)

* 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

docs/Materials.md.html

@@ -932,6 +932,36 @@ material {
 }
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+### General: featureLevel
+
+Type
+:    `number`
+
+Value
+:     An integer value, either 1 or 2. Defaults to 1.
+
+      Feature Level    |        Guaranteed features
+:----------------------|:---------------------------------
+1                      | 9 textures per material
+2                      | 12 textures per material, cubemap arrays, ESSL 3.10
+[Table [featureLevels]: Feature levels]
+
+Description
+:     Sets the feature level of the material. Each feature level defines a set of features the
+      material can use. If the material uses a feature not supported by the selected level, `matc`
+      will generate an error during compilation. A given feature level is guaranteed to support
+      all features of lower feature levels.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ JSON
+material {
+    featureLevel : 2
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Bugs
+:     `matc` doesn't verify that a material is not using features above its selected feature level.
+
+
 ### General: shadingModel
 
 Type