This uses a new strategy to fix #18998. The server-side ranks now always
render on a black background while still ensuring that the the
background is preserved in cases where the server-side ranks show the
rendering results (e.g. tile display).

This commit removes logic from vtkPVClientServerSynchronizedRenderers
and vtkIceTSynchronizedRenderers that forced the server side background
to be black. This is now gracefully handled in
vtkPVSynchronizedRenderer using `vtkSynchronizedRenderers::FixBackground`.

Furthermore, vtkSynchronizedRenderers now longer keeps separate buffers
for full res and subsampled images. Hence updating
vtkIceTSynchronizedRenderers (and others) accordingly.

Fixes #19081.