Optimize Rendering by reducing memory usage

[gofal]

What is, according to you, missing from fo-dicom? Please describe.
The rendering has been reworked in PR #1591, but there are many places where memory is allocated temporary. Some of them can be avoided, MemoryPool could be used, Pixeldata has a PinnedArray, which is not necessary, etc
The render method currently always creates a new image instance. It would be better to have an additional method that allows rendering into an existing image instance (e.g. when re-rendering an image after the windowing-option was changed)