(bugfix) Xnest: fix upstream visuals retrieval and visual reporting

As upstream might report a huge number of visuals with same parameters, we have a deduplication mechanism.

This code has a bug: instead of just skipping duplicates, it completely breaks off once the first duplicate is found - leading to only one depth supported.

Fixes: adc7f9a4 Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net