modernfmt: JXL/HEIF/AVIF loader backends, conditional (qt0)

M5 (task qt0): modern format backends.

- src/loader/backends/jxl.c: libjxl decoder → RGBA → GdkMemoryTexture.
- src/loader/backends/heif.c: libheif decoder → interleaved RGBA →
  GdkMemoryTexture (also handles AVIF when libavif is absent).
- src/loader/backends/avif.c: libavif decoder (written; auto-disabled when
  libavif is not installed).
- src/loader/loader.c: BACKENDS array conditionally includes the backends
  via -DHAVE_JXL/HAVE_AVIF/HAVE_HEIF; pixbuf remains last (fallback).
- meson.build: conditional backend sources + c_args + extra deps.
- tests: assert_unsupports updated to assert NULL only (any error, works
  with or without the specific backend compiled in).
- Minimal build (all disabled) + full build (jxl+heif) both 14/14 green,
  ASan clean.

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