Can't thumbnail some webps

[Sorunome]

While the example wepb is an animated one, it appears to be an issue with thumbnailing some webps

To Reproduce

send the following image into a room: https://mathiasbynens.be/demo/animated-webp-supported.webp

Describe the bug

Nov 17 21:14:25 sorunome media_repo[4180]: INFO[2020-11-17 20:14:25.323 Z] Received request                              contentLength=0 contentType="" host=sorunome.de method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd usingForwardedHost=false
Nov 17 21:14:25 sorunome media_repo[4180]: INFO[2020-11-17 20:14:25.323 Z] Host is valid - processing request            contentLength=0 contentType="" host=sorunome.de method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd usingForwardedHost=false
Nov 17 21:14:25 sorunome media_repo[4180]: INFO[2020-11-17 20:14:25.323 Z] Getting media record from database            allowRemote=true contentLength=0 contentType="" host=sorunome.de mediaId=de1ac13908331fe68c4e34edd8732483dd041edd method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 requestedAnimated=true requestedHeight=800 requestedMethod=scale requestedWidth=800 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd server=sorunome.de usingForwardedHost=false
Nov 17 21:14:25 sorunome media_repo[4180]: INFO[2020-11-17 20:14:25.323 Z] Getting thumbnail record from database        allowRemote=true contentLength=0 contentType="" host=sorunome.de mediaId=de1ac13908331fe68c4e34edd8732483dd041edd method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 requestedAnimated=true requestedHeight=800 requestedMethod=scale requestedWidth=800 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd server=sorunome.de usingForwardedHost=false
Nov 17 21:14:25 sorunome media_repo[4180]: INFO[2020-11-17 20:14:25.323 Z] Thumbnail does not exist, attempting to generate it  allowRemote=true contentLength=0 contentType="" host=sorunome.de mediaId=de1ac13908331fe68c4e34edd8732483dd041edd method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 requestedAnimated=true requestedHeight=800 requestedMethod=scale requestedWidth=800 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd server=sorunome.de usingForwardedHost=false
Nov 17 21:14:25 sorunome media_repo[4180]: INFO[2020-11-17 20:14:25.324 Z] Generating thumbnail                          allowRemote=true contentLength=0 contentType="" host=sorunome.de mediaId=de1ac13908331fe68c4e34edd8732483dd041edd method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 requestedAnimated=true requestedHeight=800 requestedMethod=scale requestedWidth=800 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd server=sorunome.de usingForwardedHost=false
Nov 17 21:14:25 sorunome media_repo[4180]: WARN[2020-11-17 20:14:25.324 Z] Returning cached reply from resource handler for resource ID thumbnail_sorunome.de_de1ac13908331fe68c4e34edd8732483dd041edd_800_600_scale_true
Nov 17 21:14:25 sorunome media_repo[4180]: ERRO[2020-11-17 20:14:25.324 Z] Unexpected error locating media: webp: error decoding thumbnail: webp: non-Alpha VP8X is not implemented  allowRemote=true contentLength=0 contentType="" host=sorunome.de mediaId=de1ac13908331fe68c4e34edd8732483dd041edd method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 requestedAnimated=true requestedHeight=800 requestedMethod=scale requestedWidth=800 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd server=sorunome.de usingForwardedHost=false
Nov 17 21:14:25 sorunome media_repo[4180]: INFO[2020-11-17 20:14:25.324 Z] Replying with result: *api.ErrorResponse &{Code:M_UNKNOWN Message:Unexpected Error InternalCode:M_UNKNOWN}  contentLength=0 contentType="" host=sorunome.de method=GET queryString="height=800&method=scale&width=800" remoteAddr="2a02:8071:8d8:f00:e2ff:201d:ad16:33d" requestId=REQ-1866 resource=/_matrix/media/r0/thumbnail/sorunome.de/de1ac13908331fe68c4e34edd8732483dd041edd usingForwardedHost=false

The important part seems to be webp: non-Alpha VP8X is not implemented

[Sorunome]

The issue seems to be with the source image being an animated webp and lots of stuff being funky with that.

An idea was to revert to an external tool if that fails, however ffmpeg doesn't do animated webps yet and imagemagick only does them as of 7.0 but in many debian repos is still 6.9 grumble grumble