Move SkComposeImageFilter into its own {.h, .cpp} files

gm/testimagefilters.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkCanvas.h"
 #include "SkColorFilter.h"
 #include "SkColorPriv.h"
 #include "SkShader.h"
 
 #include "SkBlurImageFilter.h"
 #include "SkColorFilterImageFilter.h"
+#include "SkComposeImageFilter.h"
 #include "SkMergeImageFilter.h"
 #include "SkOffsetImageFilter.h"
 #include "SkTestImageFilters.h"
 
 #define FILTER_WIDTH    SkIntToScalar(150)
 #define FILTER_HEIGHT   SkIntToScalar(200)
 
 static SkImageFilter* make0() { return new SkDownSampleImageFilter(SK_Scalar1 / 5); }
 static SkImageFilter* make1() { return new SkOffsetImageFilter(SkIntToScalar(16), SkIntToScalar(16)); }
 static SkImageFilter* make2() {
@@ skipping 109 matching lines @@
     }
 
 private:
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* MyFactory(void*) { return new TestImageFiltersGM; }
 static skiagm::GMRegistry reg(MyFactory);