WebGLTextures: Add support for CubeTexture with compressed textures.

src/renderers/webgl/WebGLTextures.js

@@ -412,7 +412,7 @@ function WebGLTextures( _gl, extensions, state, properties, capabilities, utils,
 
 			_gl.pixelStorei( _gl.UNPACK_FLIP_Y_WEBGL, texture.flipY );
 
-			var isCompressed = ( texture && texture.isCompressedTexture );
+			var isCompressed = ( texture && ( texture.isCompressedTexture || texture.image[ 0 ].isCompressedTexture ) );
 			var isDataTexture = ( texture.image[ 0 ] && texture.image[ 0 ].isDataTexture );
 
 			var cubeImage = [];