BUG: migrate to QOpenGLWidget

Summary

In Monteverdi's ImageViewWidget, migrate QGLWidget to QOpenGLWidget introduced in Qt 5.4.

Rationale

The old QGLWidget is obsolete in Qt 5.

After this patch the performance of image browsing in Monteverdi on my machine is increased dramatically. I can browse an image smoothly while it was very slow before.

Please see the Qt Documentation for QOpenGLWidget for details on the differences between the two.

Closes #1626 (closed). Probably closes #1796 (closed).

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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added feature label

added bug label

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Tested on Ubuntu 18.04 (superbuild) : it works fine, whereas it was very slow with Monteverdi OTB 7.0

mentioned in merge request !652 (merged)

mentioned in commit e8b88b79

merged

changed milestone to %7.1.0