fix: Some gifs not thumbnailing correctly

fix missing zero-check to disable thumbnailing size limits

Add X-Robots-Tag header

Fix apng blend / disposal

The spec is a bit unclear whether this is even valid for things like images, however the parameter doesn't really make sense for non-text data anyways.

Synapse currently (for better or worse) treats the presence as a charset as a completely different media type and thus refuses to thumbnail it. 

assume no orientation on bad exif information

Request remote media with r0 instead of v1

Fixes https://github.com/turt2live/matrix-media-repo/issues/267

Fixes https://github.com/turt2live/matrix-media-repo/issues/272