fix(oiio): don't advertise HEIC/HEIF as writable without an HEVC encoder

[mcoliver]

Summary

IOoiio advertises HEIC/HEIF (heic, heif, hif) as writable whenever OpenImageIO has a heif output plugin. But writing HEIC needs an HEVC encoder, which our Windows dependencies deliberately don't include (no x265; see AGENTS.md §1.4). So on Windows utvio -formats lists HEIC as (Read, Write) and the write then fails with OIIO: Unable to open "...heic" for writing: Unknown error (#67).

Proposal

• When the plugin loads, find out whether HEVC encoding is actually available. For example, do a one-time 1×1 write of a HEIC image to a temporary file, or query libheif directly (heif_have_encoder_for_format(heif_compression_HEVC)) if we link it. Register heic/heif/hif as read-only if not.
• AVIF (avif) uses the AV1 encoder and should stay writable when available; probe it separately.
• Then drop the heic/heif/hif Windows xfail entries in src/test/CliSmokeTest/format_roundtrip.py. The test only round-trips formats advertised as writable.

Related: #67, AGENTS.md §1.4.

Code of Conduct

• I agree to follow this project's Code of Conduct

[mcoliver]

Fixed in 2026.10 by #84. IOoiio now test-encodes HEIC and AVIF when it loads and registers them as read-only when the encoder is missing. CI shows the result on each platform: on Windows HEIC is no longer written through OpenImageIO (the round-trip test reports it as skipped, "needs an encoder that is not installed"), and on Linux, where libheif has an encoder, heic / heif / hif still round-trip.