Remove clipToImageBuffer from SourceAlpha and feComposite

Remove clipToImageBuffer from SourceAlpha and feComposite
https://bugs.webkit.org/show_bug.cgi?id=78355

Reviewed by Nikolas Zimmermann.

The implementation of clipToImageBuffer is inefficient on
non-mac platforms, so we would benefit if remove it.

Existing tests cover this feature.

* platform/graphics/filters/FEComposite.cpp:
(WebCore::FEComposite::platformApplySoftware):
* platform/graphics/filters/SourceAlpha.cpp:
(WebCore::SourceAlpha::platformApplySoftware):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107818 268f45cc-cd09-0410-ab3c-d52691b4dbfc