Add a warning or change behaviour for clip-paths not being the first item when exporting to AVD

[KieronQuinn]

Currently Shape Shifter (beta) allows you to export an AVD with a clip-path as any child of a group or the root vector. AVD doesn't seem to support this, the clip-path must be the first item of the child or it will not work. It's not documented anywhere, but I hit the issue when working on an animation, and was pointed to the solution by this stack overflow post.

Example:

Renders fine in Shape Shifter, but the mask will not be rendered in AVD

Renders fine in both Shape Shifter and AVD

Interestingly, Shape Shifter stable seems to handle the issue better, but still not perfectly. If the mask is placed last, the render looks like this:

Placed first, it looks like this:

The render for it being placed first is incorrect, but hitting play and then restarting the animation will make it render correctly:

Here is the .shapeshifter animation I was using for the above, though it should work with any vector with a clip-path: vector (12).zip