[COLRv1] Feature detection define?

[Dominik Röttsches]

Hi Werner,

Once COLRv1 is merged, in Chromium (and Skia test bots) we are most
probably going to roll to FreeType tip-of-tree as usual. When building
support for COLRv1 rasterization into Skia, we would need a way to
determine whether the FreeType library headers that we are building
against support COLRv1 types and API, so that we can move the
implementation in Skia behind #ifdef's when building against FreeType
versions that do not support it.

Ben and I have been thinking a bit about this. What I'd like to
propose is adding a derived #define to ftoptions.h that defines
TT_SUPPORT_COLRV1 if TT_CONFIG_OPTION_COLOR_LAYERS is defined.

The proposed patch looks like this
https://github.com/drott/freetype2-colr/commit/a4c2f36b1f6a7a6799248d7e13745f290fa24a69

This would only be needed as long as the feature is not officially
released, since then we could use version check macros to achieve the
same.

What do you think about this approach?

Dominik