Raytracing progress bar appears to be broken

[christianbrinch]

The progress bar used to track the ray-tracing progress. With version 1.5 it seems to stay at 0 until the ray-tracing is done (and then jump to 100%).
I have only tried continuum ray-tracing so far, so I don't know if this bug also affects line ray-tracing too.

[smaret]

I confirm that the bug also affects the line ray-tracing.

@imckstewart: would you mind having a look at this?

[allegroLeiden]

Fixed by 18bf031, branch message_fixes.