Fix color conversion from RGB to xyY

[ix-bruiz]

Description

Fix color conversion from RGB to xyY space.

Tests

Added testcase for xyY conversion in transformc testsuite.

Checklist:

• I have read the contribution guidelines.
• I have previously submitted a Contributor License Agreement.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

I believe this need an update of test reference output.

[ix-bruiz]

Indeed! Didn't notice it wasn't automatically updated! Should be good now.

By the way, it was pretty hard to make the test system work on windows (I didn't try it on Linux), and we had to dig pretty deeply into it to understand how everything was setup (and fix the windows related issues). We'll make a PR to fix this on Windows, but it would also be great to have a small documentation on how to properly contribute to the tests (maybe this already exists, but I couldn't find it).

[ix-bruiz]

I also pushed a fix regarding test compilation on Windows, where casting nullptr to intptr_t is invalid.

[lgritz]

LGTM, thanks for the fix!