Add JNA direct mapping to highly used methods

[neilcsmith-net]

For performance reasons it might be useful to support JNA direct mapping on some highly used methods (eg. Sample, Buffer methods used in AppSink callbacks). Doing some of this work before 1.0.0 may also highlight any potential issues.

[neilcsmith-net]

Preliminary work around GPointer is done. Moving to 1.1 for additional work.