[Development] Deprecation of Qt Canvas3D in 5.12

[Miller, Douglas K. CONT NAVAIR 2109, 1, N139]

I just realized from the Qt 5.12 New Features posting that Qt Canvas 3D will be deprecated in 5.12.  I'm working on my second application that uses it now and I'm concerned that the decision to deprecate it has been made too hastily.  The real advantage I see in using it is that it offers a three.js development environment which is higher level, has plethora of code that can be easily ported to the Qt Canvas 3D, is well documented, has a lot of books published for it.  It seems to be the most productive way to do 3D graphics programming in Qt unless there is a need for higher performance.  Qt 3D has several advantages over Canvas 3D, but is disadvantaged in all the areas where I mentioned that Canvas 3D/Three.js excels.  So, can we please keep longer?