Implement support for array textures in GL driver

note: feature detection was not implemented in the legacy driver, but the code itself probably works.

irr/include/EDriverFeatures.h

@@ -132,6 +132,9 @@ enum E_VIDEO_DRIVER_FEATURE
 	//! Support for multisample textures.
 	EVDF_TEXTURE_MULTISAMPLE,
 
+	//! Support for 2D array textures.
+	EVDF_TEXTURE_2D_ARRAY,
+
 	//! Only used for counting the elements of this enum
 	EVDF_COUNT
 };