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/SkPaint.h b/include/core/SkPaint.h
index 4defe8b..0e68f03 100644
--- a/include/core/SkPaint.h
+++ b/include/core/SkPaint.h
@@ -29,6 +29,7 @@
 class SkMatrix;
 class SkPath;
 class SkPathEffect;
+struct SkPoint;
 class SkRasterizer;
 class SkShader;
 class SkTypeface;