Merge pull request #143 from Unity-Technologies/dev_fix_mac_ios_color_issue

fix srgb color issue on mac/ios

engine/src/shell/platform/unity/darwin/ios/unity_surface_manager.mm

 {
   //Constants
   const int ConstCVPixelFormat = kCVPixelFormatType_32BGRA;             
-  const MTLPixelFormat ConstMetalViewPixelFormat = MTLPixelFormatBGRA8Unorm_sRGB;
+  const MTLPixelFormat ConstMetalViewPixelFormat = MTLPixelFormatBGRA8Unorm;
   const GLuint ConstGLInternalFormat = GL_RGBA;        
   const GLuint ConstGLFormat = GL_BGRA_EXT;
   const GLuint ConstGLType = GL_UNSIGNED_INT_8_8_8_8_REV;

engine/src/shell/platform/unity/darwin/macos/unity_surface_manager.mm

 void* UnitySurfaceManager::CreateRenderTexture(size_t width, size_t height)
 {
   //Constants
-  const MTLPixelFormat ConstMetalViewPixelFormat = MTLPixelFormatBGRA8Unorm_sRGB;
+  const MTLPixelFormat ConstMetalViewPixelFormat = MTLPixelFormatBGRA8Unorm;
   const int ConstCVPixelFormat = kCVPixelFormatType_32BGRA;
   const GLuint ConstGLInternalFormat = GL_SRGB8_ALPHA8;
   const GLuint ConstGLFormat = GL_BGRA;