Note the following use-case is broken in 3.3.3, and should work in 4.0:

• Create two vector layers
• Create a vector object on each layer with a different pattern
• Save and load

In 3.3, this will load the same pattern because the first encountered pattern image will be added to the pattern store as picture1; and since the second one, too, is named picture1, that will overwrite the first pattern.

SVG allows BASE64 loading/saving of image data, so either we decide upon a more advanced way of handling patterns, or we just embed the pattern images.