WIP: Embedded textures

When a texture is embedded in the blendfile and its path does not exist on disk (parent folders), then dump_image fails to save it on disk. This quick fix create the parent folders before saving the file.

However it does not fix anything if the image is embedded but has no file path. Also I wonder if it is a valid strategy to create folders on the user's disk and writing the image. If the image is packed in the file and not on disk, there is probably a good reason.

I have a question before trying something more robust: Why not just storing (and loading from clients) the pixels array of the bpy.Image ? Right now you seem to transfers the file content and write it in the cache folder of clients in order to use it as a source. I have the impression it is unecessary to use a file, but since I'm not very familiar with the image API of Blender, maybe it is impossible to directly set the pixels array.