fix(heif): Can not output AVIF when libheif has no HEVC support (AcademySoftwareFoundation#5013)

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.

Not really any practical way to automatically test this.

Signed-off-by: Brecht Van Lommel <brecht@blender.org>
Signed-off-by: Vlad (Kuzmin) Erium <libalias@gmail.com>
Signed-off-by: Vlad <shaamaan@gmail.com>

Author: brechtvl

src/heif.imageio/heifoutput.cpp

@@ -49,7 +49,9 @@ class HeifOutput final : public ImageOutput {
     std::unique_ptr<heif::Context> m_ctx;
     heif::ImageHandle m_ihandle;
     heif::Image m_himage;
-    heif::Encoder m_encoder { heif_compression_HEVC };
+    // Undefined until we know the specific requested encoder, because an
+    // exception is thrown if libheif is built without support for it.
+    heif::Encoder m_encoder { heif_compression_undefined };
     std::vector<unsigned char> scratch;
     std::vector<unsigned char> m_tilebuffer;
     int m_bitdepth = 0;
@@ -140,12 +142,13 @@ HeifOutput::open(const std::string& name, const ImageSpec& newspec,
         m_himage.add_plane(heif_channel_interleaved, newspec.width,
                            newspec.height, m_bitdepth);
 
-        m_encoder      = heif::Encoder(heif_compression_HEVC);
         auto compqual  = m_spec.decode_compression_metadata("", 75);
         auto extension = Filesystem::extension(m_filename);
         if (compqual.first == "avif"
             || (extension == ".avif" && compqual.first == "")) {
             m_encoder = heif::Encoder(heif_compression_AV1);
+        } else {
+            m_encoder = heif::Encoder(heif_compression_HEVC);
         }
     } catch (const heif::Error& err) {
         std::string e = err.get_message();