webp-container-spec: add prose for rendering process
webp-container-spec: add prose for rendering process

this is given in addition to the pseudocode, which has been corrected in
the process

based on comments in:
https://datatracker.ietf.org/doc/draft-zern-webp/ballot/#draft-zern-webp_lars-eggert

Bug: webp:448
Change-Id: I96bc063c2a71572ff61609a731a9c4e8edc2b971

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp