iv: tolerate/display partially-written exr files (originally #1191).

The idea is to make sure that iv can accept a partial file, meaning: one that opens, but some scanlines/tiles succeed when read and others fail, it should be tolerant of this, and optionally may want a display mode where it's clear which tiles are missing versus black.

The iv side is probably straightforward. The question is how many things you'll bump into upstream (in ImageBuf? ImageCache? ImageInput? OpenEXR's ImageInput implementation?) that might stand in your way and need some adjustment to tolerate the missing parts of the file. Those will need to get fixed.

Start with iv and trace partial-read handling through ImageBuf, ImageCache, and ImageInput, including the OpenEXR ImageInput implementation named in the issue. Determine which layers reject missing scanlines or tiles and whether a display mode should distinguish missing tiles from black pixels. Done means partially written EXR files open and render available data without failure, with any agreed missing-tile indication covered by tests.

Written by the indexing model from the issue text.