Fix build of PhysicsDebugRenderer::Color.

For some reason, MSVC2013 doesn't want to do brace initialization
on the trivial POD struct Color, so I had to add an actual constructor.
Add comment explaining the NOMINMAX definition.
Fix winsock header conflict.

From the comment I left in the CMakeLists:

Several Qt headers include windows.h, which includes winsock.h by
default. The enet headers include winsock2.h. winsock.h and winsock2.h
can't both be included, since they have conflicting declarations.
Thus, to be able to include Qt headers and enet headers in
the same compilation unit, we define WIN32_LEAN_AND_MEAN so that
windows.h doesn't include winsock.h.

This also disables other windows headers and features,
which should speed up compilation.