Also finally move freetype to its own env and disable warnings for it.
Still needs some work to fix the awkward situation of the freetype and
svg modules used in scene/ and editor/ respectively.
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
It was likely copy pasted by mistake from the freetype module,
which is more complex due to its usage in the core of the engine.
The SVG module does not seem to need any special treatment,
apart from linking in the main env so that editor can access it.
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
- The Windows, UWP, Android (on Windows) and Linux builds are
tested with Scons 3.0 alpha using Python 3.
- OSX and iOS should hopefully work but are not tested since
I don't have a Mac.
- Builds using SCons 2.5 and Python 2 should not be impacted.
