initial Dreamcast port

[asiekierka]

What works:

• Everything, as long as you load MegaZeux off a CD. (The file listing for "/" doesn't exist, it should list "cd" and SD card directories, if any for the latter.)

What could be done:

• Accelerated hardware renderer, but I'll wait until this port has a single-digit number of actual users for that.
• Script to take the ELF and build a CD image with chosen files, but it requires a decently high number of obscure, externally compiled tools in addition to the KallistiOS toolchain. (I'm not sure what the Common Practices for homebrew distribution are in the Dreamcast scene. Is there a way to load ELFs or binaries off an SD card? Does this preserve argv, if so?)
• Preserving/detecting correct argv, see above.

What I still want to do before merging:

• Dreamcast keyboard support.
• Find someone who could help testing this on real hardware and hopefully explain what the preferred means of Dreamcast homebrew distribution are.

NOTE: The PR is fine for merging either way; it is fully functional.

For in-emulator testing: You have to use Redream, Reicast does not emulate direct video memory writes yet.

[AliceLR]

TODO:

• The framebuffer renderer can go apparently. Not touching this until I have a working debug setup or working build.
• Analog stick/triggers would be nice. Not touching this until I have a working debug setup or working build.
• event.h is unused?
• Some style cleanup I left for later.