add bench for building mipmaps

gyp/bench.gypi

 {
   'include_dirs': [
     '../src/core',
     '../src/effects',
     '../src/gpu',
     '../src/utils',
     '../tools',
   ],
   'dependencies': [
     'etc1.gyp:libetc1',
@@ skipping 54 matching lines @@
     '../bench/LineBench.cpp',
     '../bench/MagnifierBench.cpp',
     '../bench/MathBench.cpp',
     '../bench/Matrix44Bench.cpp',
     '../bench/MatrixBench.cpp',
     '../bench/MatrixConvolutionBench.cpp',
     '../bench/MemcpyBench.cpp',
     '../bench/MemoryBench.cpp',
     '../bench/MemsetBench.cpp',
     '../bench/MergeBench.cpp',
+    '../bench/MipMapBench.cpp',
     '../bench/MorphologyBench.cpp',
     '../bench/MutexBench.cpp',
     '../bench/PatchBench.cpp',
     '../bench/PatchGridBench.cpp',
     '../bench/PathBench.cpp',
     '../bench/PathIterBench.cpp',
     '../bench/PathUtilsBench.cpp',
     '../bench/PerlinNoiseBench.cpp',
     '../bench/PictureNestingBench.cpp',
     '../bench/PicturePlaybackBench.cpp',
@@ skipping 15 matching lines @@
     '../bench/StrokeBench.cpp',
     '../bench/TableBench.cpp',
     '../bench/TextBench.cpp',
     '../bench/TileBench.cpp',
     '../bench/VertBench.cpp',
     '../bench/WritePixelsBench.cpp',
     '../bench/WriterBench.cpp',
     '../bench/XfermodeBench.cpp',
   ],
 }