fix(heif): Can not output AVIF when libheif has no HEVC support

[brechtvl]

Description

Initializing the encoder to heif_compression_HEVC by default throws an exception when libheif was built without HEVC support, which prevents it from being used entirely even if there is AVIF support.

So delay that initialization until we are sure which encoding we want.

Tests

Not really any practical way to automatically test this.

Checklist:

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[lgritz]

LGTM!