Update SkFlattenable buffers to be more modular.

This CL is an effort to stage the conversion to named
parameters for all SkFlattenable commands. This particular
stage only does the following two things...

1. Move flattenable buffers from SkFlattenable.h into
   their own header.
2. Update and Add new read write methods for better clarity
   and convenience.

BUG=

Review URL: https://codereview.appspot.com/6448095

git-svn-id: http://skia.googlecode.com/svn/trunk@4980 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/core/SkColorFilter.h b/include/core/SkColorFilter.h
index 76a0c93..4c5eee7 100644
--- a/include/core/SkColorFilter.h
+++ b/include/core/SkColorFilter.h
@@ -14,6 +14,8 @@
 #include "SkFlattenable.h"
 #include "SkXfermode.h"
 
+class SkBitmap;
+
 class SK_API SkColorFilter : public SkFlattenable {
 public:
     SK_DECLARE_INST_COUNT(SkColorFilter)