ImageDecoderExt is named incorrectly

[HeroicKatora]

Lifting this from this review which discussed some changes to the ImageDecoder. One thing that came up was that efficient decoding of parts of an image is not particularly ergonomic to use nor to implement. The name of the trait ImageDecoderExt also obscures the fact that it is used for this purpose.

The typical *Ext naming is for traits that purely extend an interface and are implementation for all T: BaseTrait, so this is also a confusing naming.

This is a very good point. Any ideas for alternate names? Perhaps ImageDecoderSeekable or something?

ImageSeek may be a bit terse but not inaccurate.