Mark Canvas3D module as deprecated

Mark qtcanvas3d module as deprecated in the 5.12 LTS version. Commenting round on Qt Development mailing list produced 3x+1's and no objections.

We’ve maintained the Canvas3D (a WebGL like API for Qt Quick JavaScript) module with very minimal effort, but when we look at our future it is clear we need to start supporting multiple low level APIs in Qt’s graphics stack and make our graphics stack less OpenGL centric. This was discussed in the Qt Contributor’s Summit 2018 and has been “in the air” for some time now among developers of Qt 3D, Qt 3D Studio and Qt Quick. Qt will of course still support OpenGL, but it’s not going to be the only API we need to support.

To this end Canvas3D with it’s WebGL API that maps nearly 1:1 to OpenGL is no longer a simple API to maintain going in to the future. It would require a major rewrite and a lot more involved design work to map it to new APIs (e.g. Vulcan, DX 12 or Metal). Also there hasn’t been a big uptake in the customers or in the community to use this QML API or the ported three.js javascript 3D engine versions. All in all Qt’s graphics stack development effort is better spent in improving Qt 3D, Qt 3D Studio and Qt Quick, adding enablers in these modules to support new graphics APIs etc.