fix(KTX2Loader): mips, cube, p3 (pmndrs#329)

Author: CodyJasonBennett

src/_polyfill/CompressedCubeTexture.js

@@ -0,0 +1,14 @@
+import { CompressedTexture, CubeReflectionMapping } from 'three'
+
+class CompressedCubeTexture extends CompressedTexture {
+  constructor(images, format, type) {
+    super(undefined, images[0].width, images[0].height, format, type, CubeReflectionMapping)
+
+    this.isCompressedCubeTexture = true
+    this.isCubeTexture = true
+
+    this.image = images
+  }
+}
+
+export { CompressedCubeTexture }