[opus] support free WEBP images in Opus files

[Timothy B. Terriberry]

Emily Bowman wrote:
> Oops, meant to add, there's absolutely no restrictions on the format of 
> any picture block, either, and opusenc will already just add whatever 
> gets passed in without even trying to verify anything about the file 
> format. That's up to tagging and playback software to support; 

Just as a minor point of correction, if you pass in a 
METADATA_BLOCK_PICTURE comment with an image/png, image/gif, or 
image/jpeg MIME type, opusenc will attempt to verify that the additional 
parameters like width, height, color depth, and palette size match what 
it finds in the file. That's merely a sanity check to reduce the 
likelihood that opusenc will create broken files, though.

If the MIME type is something else or it fails to extract those 
parameters from the file according to the format specified by the MIME 
type, then no validation is done, and it will just pass through the data 
as you say.