Support for multiple images with a mask

[mykelk]

In TikzPictures.jl there is some code in Base.writemime() that will search and replace some of the text in the svg so that things show up correctly when the svgs are combined together in something like an IJulia notebook. It does some replacement for things like glyph, clip, image, linear, etc. but I think it will also need to do something similar for "mask". The mask tag is important for controlling opacity.