[Branch Migration] Develop

This is the second of 3 pull requests applying our branch migration strategy:

https://github.com/mono/MonoGame/wiki/Branch-Migration-Strategy

This step merges `develop3d` over the `develop` branch.   This makes the two branches identical at this point in time.

The commits and file changes do not need any validation or fixing.  They are all changes that have been tested and accepted into `develop3d`.