Improve texture file support

1) Fix support for non-compressed textures in texture files (e.g. ktx). Needs fix in qtbase

2) Avoid duplication of the GLformatId->RHIformatId mapping function. qtdeclarative has a proper one internally; in quick3d this is achieved with a duplicated and less complete glformat->ssgformat->rhiformat conversion
One idea is to make general (static) api for this in QTextureFileData, perhaps both the mapping function to Rhi and the meta info query: is format compressed or not, is it srgb or not, is it opaque or not.
Ref. https://codereview.qt-project.org/c/qt/qtdeclarative/+/303517

3) Add support for .dds files? (-> qtbase task)