Fix compilation and debugging on Windows.

Core code has too many #define dependencies on interface-specific defines. This means that it's currently not possible to safely separate the core and interface code into different contexts. The core code should be refactored to accomadate for this because we should not have any interface-specific code in core in the first place.

This reverts the static library SRB2Core from a7135094 and instead adds the core sources to the SRB2SDL2 target directly.

So frustrating...