GlStackedColumnChart: Fix tooltip item order for tiled presentation

What does this MR do?

Fixes the tooltip item order in GlStackedColumnChart to account for the tiled presentation mode. Previously, tooltip items were always displayed in inverted order to match stacked columns from top to bottom, but this didn't make sense for tiled presentation where columns are displayed side-by-side. (For more context, see gitlab-org/gitlab!213207 (comment 2897838188)).

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• GitLab: gitlab-org/gitlab!213965 (closed)
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