HYDRA-13 throw error on ImageMask

We do not currently handle ImageMask because the tmpCanvas fill and size/other is only known client-side. Until we make it work, this will draw the page server-side.

I did a simple test of pushing the stencil buffer into the draw instructions with a for loop that applies it to the tmpCanvas at draw-time, but this ran into some of the scaling issues like we see with patterns and it was slow with very large stencil images (because it walked through each pixel to redraw the tmpCanvas with transparency in the stencil-excluded pixels): http://screencast.com/t/U2Fjk4MfB3zf

test with pdf-client-services: https://github.com/bovardtiberi-wf/pdf_client_service/tree/dah/HYDRA_13_imagemask_nonsmithy (https://ci.webfilings.com/build/8234)

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