Don't fail SkMergeImageFilter if one of the inputs are empty
If one of the inputs to a SkMergeImageFilter was clipped away or
otherwise caused the filterImage(...) invocation for it to return
false, the entire effect would be "failed" and return false --
regardless of if it had produced a result or not.
Instead of returning false directly if filterImage(...) for a source
returned false, consider all the inputs, and then only return false if
all of them do.
BUG=chromium:489046
Review URL: https://codereview.chromium.org/1133523006
2 files changed
