[Branch Migration] Develop3d

This is the first of 3 pull requests applying our branch migration strategy:

https://github.com/mono/MonoGame/wiki/Branch-Migration-Strategy

This first step fixes all conflicts with the `develop` branch making it easier for downstream users to eventually migrate their work to the new `develop` branch.  

While there are commits here there are no actual files changed because we are accepting the `develop3d` versions of all files.