Add contrib/colour header-only colour-conversion library (#158)

[garyo]

Part of the Colour Managed Color Parameter work (#158). Companion to #245 (the kOfxParamPropColourManagement spec change).

What

Adds contrib/colour/ofxColourConvert.h: a small, dependency-free, header-only C++17 library that converts RGB triplets between the OFX native colourspaces and ACES2065-1, for plug-in and host authors who want exact conversions without taking on a full OpenColorIO dependency. It is not a replacement for OCIO — display rendering and the display-referred / ADX / "basic" spaces are out of scope.

This is the conversion helper referenced by #245: a plug-in that declares a parameter kOfxParamColourManagementManaged receives its values in ACES2065-1, and can use this library to convert them to or from whatever working space it needs.

Every supported space reduces to code value --(transfer function)--> linear RGB --(3x3 matrix)--> AP0 and back; gamut matrices are derived at compile time (NPM + von Kries adaptation) to match the OCIO ACES config the OFX native colourspaces are based on.

Layout & integration

• contrib/colour/ofxColourConvert.h — the library, with a README.
• contrib/colour/tests/ — validation suite comparing every supported colourspace against OpenColorIO in both directions, plus round-trip and name-lookup checks.
• CMake/Conan package only the header: an INTERFACE target OpenFX::ColourConvert that installs it, plus a ColourConvert Conan component. No test dependencies leak into the core package.
• CI release tarball ships the header under openfx/contrib/colour/.
• Tests are pcons-driven (pcons fetches OpenColorIO + GoogleTest via Conan), gated behind OFX_BUILD_COLOUR_TESTS (default OFF). When on, a CTest fixture delegates to uvx pcons, so ctest runs them with zero install and no impact on the main build.

Validation

• Header compiles clean under -Wall -Wextra; all 39 colourspaces round-trip to 1e-9.
• Full Conan→CMake→CTest path: 77/77 gtest cases pass, 0 skipped (every space compared against OCIO in both directions).
• OpenFX::ColourConvert links from a consumer; cmake --install and the CI tarball both place the header.

🤖 Generated with Claude Code

[garyo]

I should note that most of this code was co-written with Claude Opus 4.8 and reviewed by me.