First and foremost I need to expand the interface to support 3D
image conversion. But the interface was not great to begin with, so this
takes the opportunity of an API break and does several things:
* The `export*()` names were rather strange and I don't even remember
why I chose that name (maybe because at first I wanted to have an
"exporter" API as a counterpart to importers?)
* In addition, there was no way to convert a compressed image to a
compressed image (or to an uncompressed image) and adding the two
missing variants would be a lot of combinations. So instead the new
convert() returns an ImageData, which can be both, and thus also
allows the converters to produce compressed or uncompressed output
based on some runtime setting, without having to implement two
(four?) separate functions for that and requiring users to know
beforehand what type of an image will be created.
* The ImageConverterFeature enum was named in a really strange way as
well, with ConvertCompressedImage meaning "convert to a compressed
image" while "ConvertCompressedData" instead meant "convert a
compressed image to a data". Utter chaos. It also all implied 2D and
on the other hand had a redundant `Image` in the name, so I went and
remade the whole thing. As mentioned above, two of the enums now mean
the same thing, and are both replaced with Convert2D.
* Finally, similarly as changes elsewhere, I took this opportunity to
get rid of std::string in the convertToFile() APIs.
Vladimír Vondruš