Using the C API provided by SkiaSharp's skia fork instead of using cairo
significantly improves performance. The API is also closer to the android
Canvas API, which makes the implementation more straightforward.
A custom GtkLayoutManager is set to these objects, which calls into the
java handlers when measure or layout is requested.
Androids onMeasure method is quite different from GTKs measure method,
because Android already defines the final size during onMeasure.
Therefore, we call onMeasure from GTKs allocate callback instead of the
measure callback.
