UI: Fix leak with paint event of volume slider

This fixes a memory leak introduced in [1] where a new QColor is not
balanced by a delete.

[1] UI: Update volume meter appearance
https://github.com/obsproject/obs-studio/commit/52ae5fc4bd518c67edd8ab94121ace05d9076892

Signed-off-by: pkv <pkv@obsproject.com>
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