Add new alphaToCoverage material property (#6606)

* Add new alphaToCoverage material property

The alphaToCoverage property lets you enable or disable alpha to coverage
in a material. More importantly it lets you overrides the behavior of
blending: masked which automatically enables alphaToCoverage.

* Update release notes

android/filament-android/src/main/cpp/Material.cpp

@@ -153,6 +153,14 @@ Java_com_google_android_filament_Material_nIsDoubleSided(JNIEnv*, jclass,
     return (jboolean) material->isDoubleSided();
 }
 
+extern "C"
+JNIEXPORT jboolean JNICALL
+Java_com_google_android_filament_Material_nIsAlphaToCoverageEnabled(JNIEnv*, jclass,
+        jlong nativeMaterial) {
+    Material* material = (Material*) nativeMaterial;
+    return (jboolean) material->isAlphaToCoverageEnabled();
+}
+
 extern "C"
 JNIEXPORT jfloat JNICALL
 Java_com_google_android_filament_Material_nGetMaskThreshold(JNIEnv*, jclass,