Viewing multiple segmentation images increases transparency even when they don't overlap

[roym899]

Sometimes you need to log multiple segmentation images instead of merging all into one, such as when masks overlap (common in, for example, amodal segmentation), or if you want to be able to hide a certain category within Rerun.

Describe the annoyance
The more segmentation images I log, the higher the transparency, even in places when they don't overlap.

Expected behavior
Either this behavior should be togglable, or it should only occur in places where segmentation images actually overlap. Generally it would be nice to have the option to set the alpha of the segmentation images manually.

Your goals
Logging segmentation images to separate entities when they overlap and so that I can show / hide segmentation images separately within Rerun.

Related to #1544