Replace float with real_t in most files, defined at the top of each file via using. Objects such as Vector3 now accept doubles as inputs, and convert to real_t internally. I've added default Vectors such as Vector3.Zero. Other misc C# improvements such as Mathf.RoundToInt(). Color continues to use float only because high precision is not needed for 8-bit color math and to keep things simple. Everything seems to compile and work fine, but testing is requested, as this is the first time I've ever contributed to Godot.
(cherry picked from commit ff97c97c93)
