Support radial gradients with a focal point.
Use standard methods for stroke style and transparency layers.

* Remove old haiku-specific code and use generic replacements.
* No functional changes.
Make clearRect clear with transparent black

* Document that this is what it must do, as shown by several tests in
the testsuite.
Remove unneeded hack.

The background doesn't overwrite the borders anymore, so we can let
WebKit handle it.
Make it possible to switch between composited and non-composited mode

* Not switchable at runtime yet, the settings would need to be exposed
* However, this shows that the non-composited mode is not working very
well anymore.