build: always build the rototiller bin

Now that there's the mem_fb backend, there's no need to disable
producing a rototiller binary in lieu of libdrm and libsdl2.

This commit also rejiggers some of the DEFAULT_VIDEO junk in
main.c to ensure it falls back on "mem" should there be no drm or
sdl2.

For now I'm going to leave the AM_CONDITIONAL junk surrounding
enabling rototiller in configure.ac, the define can just be
ignored for now.

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