Implement posterCanvas for Presentation 3.0

[joncameron]

Description

Implementing posterCanvas from the IIIF Presentation 3.0 spec.

Implementation

• posterCanvas is a hash of five keys (url, height, width, format, type) that then gets passed to DisplayContent and run through the CanvasBuilder

References

• https://iiif.io/api/presentation/3.0/#postercanvas

[kirkkwang]

@joncameron since posterCanvas has been replaced by placeholderCanvas and accompanyingCanvas, should this issue be closed or changed?

ref: IIIF/trc#8