[NUI] Make PixelBuffer & PixelData as Disposable

[NUI] Make PixelBuffer & PixelData as Disposable

Since they were BaseHandle, there was some Fatal log printed
when we try to use them at worker thread.

For example, PixelBuffer buffer = ImageLoader.LoadImageFromFile() was invalid.

This patch make them as Disposable.
So they will not be registed into Registry.

Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>