Linux implementation using ffmpeg

Without this change I get CMake errors due to use of ${FLTK_INCLUDE_DIR}
and references to the non-existent vncviewer target.

They have very different purpose, so make things easier to work
with by having multiple directories.

We won't always be on the primary monitor, so check which color space
we're actually using right now. For offscreen stuff we assume a standard
sRGB color space.

This will allow us to render more things than just the framebuffer.

We want to have other tests related to pixel conversion, so avoid
possible naming conflicts.

Add a tests directory where we can put tests programs that can
be useful during testing. Start with a speed test for the pixel
conversion routines.