Cleanup the OpenGL examples
- use nullptr (clang-tidy)
- use member-initialization
- avoid redundant checks for != nullptr when deleting a pointer
Change-Id: I3e4702690ed79e71c3e952d51ceef83b907b45b7
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>