Fix cube map demo

Change-Id: Id8d3fce0840627de0414e26d1b2a60d89cfd2fe1
Docs-Preview: https://skia.org/?cl=316942
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/316942
Commit-Queue: Nathaniel Nifong <nifong@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
2 files changed