add virtuals to optimize composing colorfilters

include/core/SkColorFilter.h

Index: include/core/SkColorFilter.h
diff --git a/include/core/SkColorFilter.h b/include/core/SkColorFilter.h
index bb628053a643527b72d876b8da88cd8302ee6f00..f8df86bec83063f72a5d63ba9ad2862991d4aeaa 100644
--- a/include/core/SkColorFilter.h
+++ b/include/core/SkColorFilter.h
@@ -98,6 +98,15 @@ public:
     virtual uint32_t getFlags() const { return 0; }
 
     /**
+     *  If this subclass can optimally createa composition with the inner filter, return it as
+     *  a new filter (which the caller must unref() when it is done). If no such optimization
+     *  is known, return NULL.
+     *
+     *  e.g. result(color) == this_filter(inner(color))
+     */
+    virtual SkColorFilter* newComposed(const SkColorFilter* inner) const { return NULL; }

[Stephen White, 2015/03/02 21:10:10]

Naming nit: maybe this could be preConcat()? (or postConcat(), whatever the
semantics are).

[reed1, 2015/03/02 21:19:19]

preConcat for matrices, compose for patheffects, shaders, colorfilters and
imagefilters. I'd rather keep the current theme (compose) for now, but if the
dox aren't clear enough, I will definitely try to improve them.

[Stephen White, 2015/03/02 21:32:52]

Hmm.. createComposed()? The factories (and various classes in Ganesh) provide a
precedent for createFoo(). OTOH SkSurface and friends provide a precedent for
newFoo(). I leave it up to you.

[reed1, 2015/03/02 21:39:11]

Yea, I was going with surface/canvas use of new as opposed to create. For sure
we should consider consolidating conventions at some point.

+
+    /**
      *  Apply this colorfilter to the specified SkColor. This routine handles
      *  converting to SkPMColor, calling the filter, and then converting back
      *  to SkColor. This method is not virtual, but will call filterSpan()