Clean up real time rendering behavior for non-windows users.

[wfpokorny]

For background see the newsgroup thread at:

http://news.povray.org/povray.animations/thread/%3C594eb2f6%241%40news.povray.org%3E/

If possible, perhaps support the feature in a way to match windows for unix builds.

Where not supported an exit with an error message of some kind would be more friendly than just not working as happens today.

[c-lipka]

What's the status with respect to the different versions (3.7.0, 3.7.1)?

[wfpokorny]

On the linux side of things the behavior in all 3.7 versions was the same. We get this warning from the parser:

Parse Warning: This rendering uses the following beta-test feature(s): real-time
raytracing render loop. The implementation of these features is likely to
change or be completely removed in subsequent beta-test versions of POV-Ray.
There is no guarantee that they will be available in the next full release
version. 

(The message text in referring to "beta-test versions" should probably be cleaned up. Perhaps strike "beta-test" ?)

A blank screen pops up, looks like the a "single-camera" render happens and no other output is generated.

Not sure what happens on windows other than Bob's report the sample scene referenced in the newsgroup thread worked for him.

[wfpokorny]

As suggested in the newsgroup thread that the real time option is distinct from the clockless (no scene reload) option, I tried the example less the real time +rtr option as :

povray +kla cube_rtr.pov

on my Ubuntu 16.04 linux system. The behavior was no different than the result from:

povray cube_rtr.pov

except that I get the following parser warning in the former:

"Parse Warning: Clockless animation: total of 200 cameras will be used."

I also tried:

povray +kla +kfi1 +kff200 cube_rtr.pov

which renders 200 uniquely named png files all at the initial clock setting (camera doesn't move) and it reloads the scene for each frame. Tested with larger mesh so parse time other than trivial. Render size set very small. Total time always roughly the sum of parsing times x what ever frame count specified.

Looks to me like the clockless animation is also not working as intended on unix.