Move SK_RELEASE define to the xcode build.
The Skia deps are always built for release, whether Plask is
built for debug or release, so always define SK_RELEASE.
Use UniquePersistent std::map piecewise_construct
C++11 to create the object in-place instead of using move semantics.
Use SkCanvas::clear instead of SkBitmap::eraseColor.
This was meant to be temporary some years ago as the API was
changing.  It is unnecessary to access the bitmap/device.
Remove the old eraseColor alias, the name is confusing anyway.