MSFT_texture_dds: behaviour for unreachable or incorrect dds files

Saw #1359 and see there is likely little motivation to maintain the MSFT_texture_dds extension but implementing requires addressing the case where the extension is supported but the .dds file itself is not available or not conforming or otherwise not loadable.

Is there a preference if in this case the loader should fall back to the regular texture source (likely) or fail (perhaps with a black texture) ?

In general, it would be helpful if the extension would follow the webp texture extension template for well defined behavior. I think we will end up doing this.

Read issue #1359 and compare the MSFT_texture_dds behavior with the webp texture extension template mentioned here. Decide and document how supported but unavailable, nonconforming, or unloadable .dds files should behave, including whether loading falls back to the regular texture source or fails.

Written by the indexing model from the issue text.