Draft: Split and refactor HW code

Based on !1516 (merged), merge that before merging this!

• Various parts of the hardware renderer are refactored
• Some old and no longer used code is removed, like the old clipping system
• Some large functions are split to smaller ones
• hw_main.c is split into multiple files: hw_sky.c, hw_things.c, hw_plane.c, hw_segs.c, hw_drawnodes.c and hw_bsp.c
• The previous hw_bsp.c file (containing map loading code) is renamed to hw_map.c to make way for the bsp handling code from hw_main.c