export target osg-qt::osgQOpenGL to cmake

[yanzixiang]

microsoft/vcpkg#44441
#53
#57
microsoft/vcpkg#26312

so many want to use osg-qt in the cmake way,
so may be we should export target osg-qt::osgQOpenGL to cmake

[dg0yt]

so many want to use osg-qt in the cmake way,

OTOH the project has no activity in three years, and it is still using Qt5.

[dg0yt]

Do not use REQUIRED with find_dependency. (You loose key benefits vs. find_package.)

[emminizer]

Do not use REQUIRED with find_dependency. (You loose key benefits vs. find_package.)

I wrote that in and was following the example from e.g. libpng (uses it for ZLIB), WebP (uses it for Threads), and vxgXchange (used for vsg). That said, I think I agree and am happy with the update, thanks for the suggestion.

I agree also about the aliases. One is sufficient and it should match.

no activity in three years, and it is still using Qt5

I've been using the older osgQt which feels like a different library than what's there now. The newest library looks like a rewrite that works but is less configurable and more tightly coupled. I'm working on patches for that locally and expect to be also looking at the Qt6 upgrade in the issues. I could create PRs to improve but am concerned no one is around to accept/reject.