On Win32, things are failing to build with messages like:
RenderableEntityItem.cpp(674,51): error C2065: 'M_PI': undeclared identifier
This is because as per Microsoft documentation, _USE_MATH_DEFINES is needed to
obtain constants like M_PI:
https://docs.microsoft.com/en-us/cpp/c-runtime-library/math-constants?view=msvc-160
It seems this worked previously due to some quirk of CMake, but stopped working
after some reordering. This change makes this definition explicit where it is needed.