Bug 964200 (Part 3 main) - Implement Filter Effects Module feDropShadow filter. r=mstange

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rename : content/svg/content/src/SVGFEGaussianBlurElement.cpp => content/svg/content/src/SVGFEDropShadowElement.cpp
rename : content/svg/content/src/SVGFEGaussianBlurElement.h => content/svg/content/src/SVGFEDropShadowElement.h
rename : dom/webidl/SVGFEGaussianBlurElement.webidl => dom/webidl/SVGFEDropShadowElement.webidl