Optimize already embedded textures

[Breush]

In yoga/model/assimp.cpp, extract_image_nodes does not extract already embedded textures, they are ignored.

Keeping the very same convention than assimp, we could just fill imageNode with:

auto imageNode = new ImageNode();
imageNode->path = "*1"; // Same convention, the 1 is the index of the textures in scene->mTextures
imageNode->bytes = /* extract bytes for scene->mTextures */;
imageNode->bytes_length = /* extract bytes length */;