update README about codec thumbnail encoding

Author: dreampiggy

README.md

@@ -269,6 +269,8 @@ let lossyAVIFData = SDImageAVIFCoder.shared.encodedData(with: image, format: .av
 
 ### Thumbnail Encoding (0.12.0+)
 
+Note: only available in aom codec encoder!
+
 + Objective-C
 
 ```objective-c

SDWebImageAVIFCoder/Classes/SDImageAVIFCoder.m

@@ -353,18 +353,21 @@ - (nullable NSData *)encodedDataWithImage:(nullable UIImage *)image format:(SDIm
     encoder->quality = quality;
     encoder->qualityAlpha = quality;
     encoder->maxThreads = 2;
-    // Check if need to scale pixel size
-    CGSize scaledSize = [SDImageCoderHelper scaledSizeWithImageSize:CGSizeMake(width, height) scaleSize:maxPixelSize preserveAspectRatio:YES shouldScaleUp:NO];
-    if (!CGSizeEqualToSize(scaledSize, CGSizeMake(width, height))) {
-        // Thumbnail Encoding
-        assert(scaledSize.width <= width);
-        assert(scaledSize.height <= height);
-        avifScalingMode scale;
-        scale.horizontal.n = (int)scaledSize.width;
-        scale.horizontal.d = (int)width;
-        scale.vertical.n = (int)scaledSize.height;
-        scale.vertical.d = (int)height;
-        encoder->scalingMode = scale;
+    // Check if need to scale pixel size, currently only available on aom
+    const char *codecName = avifCodecName(codecChoice, AVIF_CODEC_FLAG_CAN_ENCODE);
+    if (codecName && strcmp(codecName, "aom") == 0) {
+        CGSize scaledSize = [SDImageCoderHelper scaledSizeWithImageSize:CGSizeMake(width, height) scaleSize:maxPixelSize preserveAspectRatio:YES shouldScaleUp:NO];
+        if (!CGSizeEqualToSize(scaledSize, CGSizeMake(width, height))) {
+            // Thumbnail Encoding
+            assert(scaledSize.width <= width);
+            assert(scaledSize.height <= height);
+            avifScalingMode scale;
+            scale.horizontal.n = (int)scaledSize.width;
+            scale.horizontal.d = (int)width;
+            scale.vertical.n = (int)scaledSize.height;
+            scale.vertical.d = (int)height;
+            encoder->scalingMode = scale;
+        }
     }
     avifResult result = avifEncoderWrite(encoder, avif, &raw);