Alpha image size can be different from the primary one.

See:
AOMediaCodec/av1-avif#68

src/main.cpp

@@ -223,17 +223,17 @@ int _main(int argc, char** argv) {
       unsigned int elapsed = decodeImageAt(log, res, alphaID.value(), ctx, alphaImg.value());
       log.info(" Decoded: %s -> %s in %d [ms] (Alpha image)", inputFilename, outputFilename, elapsed);
       if(alphaImg.value().p.w != primaryImg.p.w || alphaImg.value().p.h != primaryImg.p.h) {
-        // TODO(ledyba-z): Can alpha image and primary image be different size?
+        // TODO(ledyba-z): It is okay to alpha image and primary image are different sizes.
         //  see: https://github.com/AOMediaCodec/av1-avif/issues/68
-        log.fatal("Alpha size (%d x %d) does not match to primary image(%d x %d).",
+        log.fatal("Currently, alpha image whose size (%d x %d) does not match to primary image (%d x %d) is not supported.",
                   alphaImg.value().p.w, alphaImg.value().p.h, primaryImg.p.w, primaryImg.p.h);
       }
       if(outputAlphaFilename.has_value()) {
         std::optional<Dav1dPicture> empty{};
         saveImage(log, outputAlphaFilename.value(), fileBox, alphaImg.value(), empty);
         log.info(" Extracted: %s -> %s (Alpha image)", inputFilename, outputAlphaFilename.value());
       }
-    }else{
+    } else {
       if(outputAlphaFilename.has_value()) {
         log.fatal("%s does not have alpha plane.", inputFilename);
       }