pull mipmap class into its own (private) header

gyp/tests.gyp

 # GYP file to build unit tests.
 {
   'includes': [
     'apptype_console.gypi',
   ],
   'targets': [
     {
       'target_name': 'tests',
       'type': 'executable',
       'include_dirs' : [
@@ skipping 59 matching lines @@
         '../tests/ImageDecodingTest.cpp',
         '../tests/InfRectTest.cpp',
         '../tests/LListTest.cpp',
         '../tests/LayerDrawLooperTest.cpp',
         '../tests/MD5Test.cpp',
         '../tests/MathTest.cpp',
         '../tests/MatrixTest.cpp',
         '../tests/Matrix44Test.cpp',
         '../tests/MemsetTest.cpp',
         '../tests/MetaDataTest.cpp',
+        '../tests/MipMapTest.cpp',
         '../tests/OSPathTest.cpp',
         '../tests/PackBitsTest.cpp',
         '../tests/PaintTest.cpp',
         '../tests/ParsePathTest.cpp',
         '../tests/PathCoverageTest.cpp',
         '../tests/PathMeasureTest.cpp',
         '../tests/PathTest.cpp',
         '../tests/PathUtilsTest.cpp',
         '../tests/PDFPrimitivesTest.cpp',
         '../tests/PictureTest.cpp',
@@ skipping 66 matching lines @@
       ],
     },
   ],
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: