Extend Texture interface to allow adding embedded textures
The texture interface currently only supports adding textures from explicit image files. In some scenarios, a texture without a respective filename to load it is required. (e.g. when embeded textures shall be loaded from a mesh file related to #116 )
As each texture is stored in the imagestore, a mapping from filename to texture id is used to manage textures. For textures without file backing an alternative could be to use dedicated internal names e.g. by prefixing the filename of file backed textures, so the image store only loads file backed textures, but ignores embedded textures.