Clean up SkImageFilter constructors.

Now that all creation of SkImageFilters goes through
factory Create() methods, there's no real reason for the
convenience constructors. Some SkImageFilter subclasses
which actually have zero DAG-able inputs were passing NULL
to the superclass constructor. This actually means 1 input,
with a NULL value, not zero inputs. This becomes more
relevant for the upcoming cache infrastructure, where this
indicates that the filter will use its src input, where in
fact some of these filters do not (they are image generators
only).

Limiting SkImageFilter to a single constructor resolves this
ambiguity.

Along the way, I removed all of the default parameters to
the constructors, since the Create methods always call them
with the full argument list.

BUG=skia:
R=reed@google.com

Author: senorblanco@chromium.org

Review URL: https://codereview.chromium.org/376953003
diff --git a/src/effects/SkXfermodeImageFilter.cpp b/src/effects/SkXfermodeImageFilter.cpp
index acb8fd3..bcfb785 100644
--- a/src/effects/SkXfermodeImageFilter.cpp
+++ b/src/effects/SkXfermodeImageFilter.cpp
@@ -21,10 +21,9 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 SkXfermodeImageFilter::SkXfermodeImageFilter(SkXfermode* mode,
-                                             SkImageFilter* background,
-                                             SkImageFilter* foreground,
+                                             SkImageFilter* inputs[2],
                                              const CropRect* cropRect)
-  : INHERITED(background, foreground, cropRect), fMode(mode) {
+  : INHERITED(2, inputs, cropRect), fMode(mode) {
     SkSafeRef(fMode);
 }