C API: Add SK_API, also documentation of an example.

src/c/sk_paint.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
+#include "SkPaint.h"
+
 #include "sk_paint.h"
 #include "sk_types_priv.h"
 
-#include "SkPaint.h"
-
 #define MAKE_FROM_TO_NAME(FROM)     g_ ## FROM ## _map
 
 const struct {
     sk_stroke_cap_t fC;
     SkPaint::Cap    fSK;
 } MAKE_FROM_TO_NAME(sk_stroke_cap_t)[] = {
     { BUTT_SK_STROKE_CAP,   SkPaint::kButt_Cap   },
     { ROUND_SK_STROKE_CAP,  SkPaint::kRound_Cap  },
     { SQUARE_SK_STROKE_CAP, SkPaint::kSquare_Cap },
 };
@@ skipping 138 matching lines @@
         MAP( HUE_SK_XFERMODE_MODE,        SkXfermode::kHue_Mode        );
         MAP( SATURATION_SK_XFERMODE_MODE, SkXfermode::kSaturation_Mode );
         MAP( COLOR_SK_XFERMODE_MODE,      SkXfermode::kColor_Mode      );
         MAP( LUMINOSITY_SK_XFERMODE_MODE, SkXfermode::kLuminosity_Mode );
         #undef MAP
         default:
             return;
     }
     AsPaint(paint)->setXfermodeMode(skmode);
 }