Merged sdlcurse.cpp and sdltiles.cpp

(See Issue #3135)
These two files were mostly identical, and prone to consistency errors as a result. I merged them into a single file (sdltiles.cpp) and removed almost all the redundancy. The source compiles the same way, only now SDLTILES should be defined alongside TILES to compile the tileset version (previously they were mutually exclusive).

Remaining issues:
~~In cursesport.cpp, there's a conditional compile flag at the top that only applies on non-windows builds~~ Fixed