Fix a couple of bugs with tiled rendering.

[brugger1]

Resolves #1739

This fixes the two bugs with tiled rendering from ticket #1739.

• Some of the tiles are blank in the image generated. This is due to a fix that wasn't quite correct where an if test was added around the code that copied a pixel from a tile to the final image so that the pixel was only copied if the depth in the tile was less than the depth in the final image. The if test included some code that incremented some indexes that should always get executed.
• The tiles that weren't blank exhibited a checker board pattern of missing image. The data had two domains where one domain had the red squares of a checkerboard and the second domain had the black squares of a checkerboard. There was a bug with the order of two loops such that the last domain only made it to the final image. The loop order was changed to loop over tiles and within tiles, loop over domains.

Here is the image generated by the reproducer in the ticket after the change.

[cyrush]

Looks great, thanks for these fixes.

One suggestion:
We have a note in the changelog that tiled rendering default was changed:
https://github.com/Alpine-DAV/ascent/blob/develop/CHANGELOG.md#changed

We should update this to say that a bug was fixed and tiled rendering defaults to on.