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.
GtkWidgets extend GInitiallyUnowned and are automatically freed when
removing from parent widget. We need to add an extra reference, to make
sure the object keeps alive as long as the java widget has a reference
to it
Makes it easier to overwrite behavior in subclasses. Have a fallback
implementation for ViewGroup.
Save some _GET_LONG_FIELD / _SET_LONG_FIELD calls by directly passing
the native pointers to and from native methods.
Adds a special treatment for the main Looper to not block in java code,
but instead return to glib managed thread loop. Timeouts in the mainloop
are now handled using g_timeout_add_full().
Also defer Activity construction, so that every thing is set up properly
when the constructor runs.
