add 'compatible' to allowed blend modes

see pdf spec 1.7 page 567 "... an additional blend mode named Compatible ... It is still recognized as a valid blend mode for the sake of compatibility but is simply treated as equivalent to Normal."
replace gco='compatible' as source-over

same as 'normal' BM, 'compatible' should be treated as gco default
'source-over' (aka normal drawing)
Merge pull request #15 from WebFilings/HYDRA-169_compatible_blend

HYDRA-169 compatible blend