Spherical harmonic transform (SHT) previews are display instead of a solid
background color before the base multires tiles are loaded. If a thumbnail is
provided, it will be displayed instead of the SHT preview. If the thumbnail is
provided as a Base64-encoded string, the SHT preview may be briefly displayed,
since image loading is asynchronous, while the SHT preview is loaded
synchronously.

Signed-off-by: Vanessa Sochat <vsochat@stanford.edu>