Fix OpacityWidget Range bug
Fix a bug with the OpacityWidget when the range was changing.
When the range was changing from a high minimum to a low maximum a bug could happen where the min was not correctly set and the opacity widget became white. This fixes it and adds a test for it.
In order to test the OpacityWidget, it is needed to add support for testing any QVTKOpenGL*Widget, this also adds it.
Fix #19090 (closed)
Edited by Joachim Pouderoux