Headless mode is too aggressive

[dirkwhoffmann]

Until now, headless mode was only used to accelerate the emulator in warp mode. This worked well because warp mode is usually enabled only during load or save operations. Combining headless mode with run-ahead mode proves it to be too aggressive. I.e., it skips the sprite drawing routine, which disables sprite collision checking, thus breaking run-ahead mode.

TODO: Either disable headless mode in the run-ahead code or make headless mode less aggressive by only skipping the last pixel synthesis stage.