HYDRA-169 compatible blend

There is another blend mode equivalent to 'normal' that is used in some annotations.

Test with hydra branch: HYDRA-169_TEST_compatible_blend (https://github.com/WebFilings/pdf_client_service/compare/master...HYDRA-169_TEST_compatible_blend)

Test with the PDFAnno*JPMC*pdf in the Box/pdf*/ directory. The first page should draw using canvas. The second through fourth pages will draw with fallback images because they use blend modes in the highlights.

![image](https://f.cloud.github.com/assets/1609679/1253057/c062c01a-2b5b-11e3-9152-c008e7636564.png)

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