This method is different from MatrixFilter, in that MatrixFilter does not require a pre-existing
filter, but LocalM does. Also change the comment to be more general, as there is no promise that
we return a different subclass, and certainly not a specific subclass.
This pattern of obj->newWithModifiers() also more closely matches the pattern in SkImage (newSubset).
BUG=skia:
Review URL: https://codereview.chromium.org/1402133002
At draw time, this filter simply concatenates the given matrix to the
CTM, and recurses on its input. The matrix is thus applied to any
upstream filter parameters and crop rects.
BUG=skia:
Review URL: https://codereview.chromium.org/1392833005
