Improve Animated WebP decoder robustness and fix warning

Only decode the first frame on macOS
Fix CGBitmapContextCreate failed issue
Duration set to 100ms if it’s lower or equal than 10ms for compatibility
Fix unused variable warning for macOS

@@ -58,8 +58,10 @@ static void FreeImageData(void *info, const void *data, size_t size) {
         return nil;
     }
     
-    int frameCount = WebPDemuxGetI(demuxer, WEBP_FF_FRAME_COUNT);
+#if SD_UIKIT || SD_WATCH
     int loopCount = WebPDemuxGetI(demuxer, WEBP_FF_LOOP_COUNT);
+#endif
+    int frameCount = WebPDemuxGetI(demuxer, WEBP_FF_FRAME_COUNT);
     int canvasWidth = WebPDemuxGetI(demuxer, WEBP_FF_CANVAS_WIDTH);
     int canvasHeight = WebPDemuxGetI(demuxer, WEBP_FF_CANVAS_HEIGHT);
     CGBitmapInfo bitmapInfo;
@@ -69,6 +71,11 @@ static void FreeImageData(void *info, const void *data, size_t size) {
         bitmapInfo = kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast;
     }
     CGContextRef canvas = CGBitmapContextCreate(NULL, canvasWidth, canvasHeight, 8, 0, SDCGColorSpaceGetDeviceRGB(), bitmapInfo);
+    if (!canvas) {
+        WebPDemuxReleaseIterator(&iter);
+        WebPDemuxDelete(demuxer);
+        return nil;
+    }
     
     NSMutableArray<UIImage *> *images = [NSMutableArray array];
     NSTimeInterval totalDuration = 0;
@@ -87,17 +94,20 @@ static void FreeImageData(void *info, const void *data, size_t size) {
         }
         
         [images addObject:image];
+        
+#if SD_MAC
+        break;
+#endif
+        
         int duration = iter.duration;
-        if (!duration) {
-            // WebP standard says duration for 0 is used for canvas updating but not showing image, but actually Chrome set this to the default 100ms duration.
-            // Some animated WebP images also create without duration, we should keep compatibility
+        if (duration <= 10) {
+            // WebP standard says 0 duration is used for canvas updating but not showing image, but actually Chrome and other implementations set it to 100ms if duration is lower or equal than 10ms
+            // Some animated WebP images also created without duration, we should keep compatibility
             duration = 100;
         }
         totalDuration += duration;
         size_t count = images.count;
-        if (count) {
-            durations[count - 1] = duration;
-        }
+        durations[count - 1] = duration;
         
     } while (WebPDemuxNextFrame(&iter));
     
@@ -143,9 +153,7 @@ static void FreeImageData(void *info, const void *data, size_t size) {
     
     CGImageRelease(newImageRef);
     
-    if (iter.dispose_method == WEBP_MUX_DISPOSE_NONE) {
-        // do not dispose
-    } else {
+    if (iter.dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
         CGContextClearRect(canvas, imageRect);
     }
     
@@ -177,9 +185,7 @@ static void FreeImageData(void *info, const void *data, size_t size) {
     
     CGImageRelease(newImageRef);
     
-    if (iter.dispose_method == WEBP_MUX_DISPOSE_NONE) {
-        // do not dispose
-    } else {
+    if (iter.dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
         CGContextClearRect(canvas, imageRect);
     }
     
@@ -199,7 +205,7 @@ static void FreeImageData(void *info, const void *data, size_t size) {
     config.output.colorspace = config.input.has_alpha ? MODE_rgbA : MODE_RGB;
     config.options.use_threads = 1;
 
-    // Decode the WebP image data into a RGBA value array.
+    // Decode the WebP image data into a RGBA value array
     if (WebPDecode(webpData.bytes, webpData.size, &config) != VP8_STATUS_OK) {
         return nil;
     }
@@ -211,7 +217,7 @@ static void FreeImageData(void *info, const void *data, size_t size) {
         height = config.options.scaled_height;
     }
 
-    // Construct a UIImage from the decoded RGBA value array.
+    // Construct a UIImage from the decoded RGBA value array
     CGDataProviderRef provider =
     CGDataProviderCreateWithData(NULL, config.output.u.RGBA.rgba, config.output.u.RGBA.size, FreeImageData);
     CGColorSpaceRef colorSpaceRef = SDCGColorSpaceGetDeviceRGB();