Update the ColourSpace example

[Guido-assim]

Do not set kOfxImageClipPropPreferredColourspaces for the output clip. Handle kOfxImageEffectActionGetOutputColourspace to set the output colourspace.

[garyo]

Tested build on Mac; builds OK. I don't have a colourspace-ready host to test against though.
The changes (adding getOutputColourspace and moving the output colourspace selection there) look fine to me at a quick glance, but @john-paulsmith will have to have the final word.

[garyo]

One small but important thing: please add a "DCO" line to your commit message. I suggest doing it by amending the latest commit and force-pushing, like this:

# make sure you're at commit ca59362 before starting this, with no changes, and then:
git commit --amend
# in your editor, add a blank line, and then "Signed-off-by: Guido Veldkamp <[email protected]>", then save.
git push -f # force-push your branch; this will update the PR automatically

In future, just use git commit -s instead of git commit to automatically add the "Signed-off-by" line.

[john-paulsmith]

Thanks for improving the example @Guido-assim, I'm going to merge it. This example doesn't do anything with the host-provided preferred output colourspace list, it would be good to include that in a future example.