fix for clip-path ids carried to another image-mask

Thanks for the component. 
I had a weird problem (which I wasn't able to reproduce in demo.html) where clip-paths with the same id (e.g. {{shape}}) were not encapsulated within the component so it was carried to another image-mask. So the shape in one would affect the other one. 

A unique maskId for each image-mask component solved that problem and I don't think it would cause any problem.