With the intention that those will eventually contain also things like
YUp / YDown, PremultipliedAlpha and such.
This commit is mostly just busywork, wiring this into [Compressed]Image,
[Compressed]ImageView and Trade::ImageData and ensuring the flags get
correctly propagated during moves and conversions. Unfortunately in case
of Trade::ImageData it meant deprecating the current set of constructor
in order to insert an ImageFlags parameter before the importer state
pointer.
The only non-trivial piece of logic is when a 2D
[Compressed]ImageView gets converted to a 3D one, then the Array bit is
implicitly dropped, as 2D arrays of 1D images are not really a thing.
Instead, it's now possible to add new flags when doing the conversion --
for example to turn a 2D image to a (single-layer) 2D array image.
Vladimír Vondruš