[do not merge] windows and render to texture

[mathisloge]

So this isn't a real PR since I don't expect these changes to merge. But there might be people outside, that needs some first ideas.

This PR builds with vcpkg (PR needs to be merges https://github.com/microsoft/vcpkg/pull/16838).

when using magnum as a platform tangram will build as a shared library which is exported correctly for dlls.

I've only tested on windows x64 with visual studio 2019.