Convert textures to OpenEXR

[matthewlow-dwa]

The current textures use .tif, .png, and .jpg image fie formats. While OpenUSD supports these formats, at least TIF requires the optional OpenImageIO plugin for OpenUSD, which may be disabled on some platforms.

To make this asset as broadly supported as possible, we should consider updating all texture files to OpenEXR, another ASWF project.