Convert YCbCr images to sRGB when decoding with Imagick

[nlemoine]

Reading pixels from an AVIF (or HEIF) fails with the Imagick driver on older ImageMagick. Decoding is fine, the first colorAt() throws.

Older ImageMagick decodes AVIF/HEIF into the YCbCr colorspace and doesn't normalize it. The Imagick ColorspaceAnalyzer has no YCbCr branch, so the match falls through to the OKLAB/OKLCH arms. Those reference Imagick::COLORSPACE_OKLAB, which doesn't exist before the ImageMagick release that added OKLAB, so analysis fails with "Undefined constant Imagick::COLORSPACE_OKLAB". Newer ImageMagick returns sRGB for the same image, so it only shows up on older builds.

Repro: decode an AVIF with the Imagick driver on an ImageMagick without OKLAB, then call colorAt() or colorspace().

Measured inside the failing environment:

ImageMagick	OKLAB defined	decoded AVIF colorspace
6.9.11-60	no	7 (COLORSPACE_YCBCR)
6.9.13-40	no	7 (COLORSPACE_YCBCR)
7.1.2	no	sRGB

Every decoded AVIF reported YCbCr regardless of the encoder (aom/libavif, vips). The colorspace enum values differ between ImageMagick majors (YCbCr is 7 on IM6, 27 on IM7), so the fix uses the Imagick::COLORSPACE_YCBCR constant, not a literal.

Fix:

1. NativeObjectDecoder converts YCbCr to sRGB on decode, next to the existing GRAY handling. It uses transformImageColorspace() (a real conversion), not setImageColorspace(), otherwise pixels come back as raw luma/chroma values. This is a no-op on newer ImageMagick where the image is already sRGB.

2. ColorspaceAnalyzer guards the OKLAB/OKLCH constants with defined() before using them, the same way ColorspaceModifier already does. Unknown colorspaces now reach the default arm instead of throwing on the missing constant.

Tests: added a regression test in NativeObjectDecoderTest that forces an image to YCbCr, decodes it, and checks the result is sRGB and colorAt() returns the original color (small tolerance, so it also catches a relabel-instead-of-convert mistake). It forces YCbCr explicitly so it runs the same on ImageMagick 6 and 7.

[olivervogel]

Thanks.

[nlemoine]

Thank you @olivervogel!