Trade,ShaderTools: add addFlags() / clearFlags() to all plugin interfaces.
There will be Flag::FlipY for images at some point, enabled by default
for compatibility with existing GL code, and so it makes sense to start
discouraging setFlags() as early as possible to avoid people resetting
the default by accident.
Also update the imageconverter, sceneconverter and shaderconverter utils
to use these instead of setFlags().
CORRADE_ASSERT(!isOpened(),"Trade::AbstractImporter::setFileCallback(): can't be set while a file is opened",);
CORRADE_ASSERT(!isOpened(),"Trade::AbstractImporter::setFileCallback(): can't be set while a file is opened",);
CORRADE_ASSERT(features()&(ImporterFeature::FileCallback|ImporterFeature::OpenData),"Trade::AbstractImporter::setFileCallback(): importer supports neither loading from data nor via callbacks, callbacks can't be used",);
CORRADE_ASSERT(features()&(ImporterFeature::FileCallback|ImporterFeature::OpenData),"Trade::AbstractImporter::setFileCallback(): importer supports neither loading from data nor via callbacks, callbacks can't be used",);