Fix the issue when monochome colorspace cause the WebP encoding failed

Just translate into RGBA first, don't need extra code to re-implements monochrome WebP encoding

Author: dreampiggy

SDWebImageWebPCoder/Classes/SDImageWebPCoder.m

@@ -885,6 +885,12 @@ - (nullable NSData *)sd_encodedWebpDataWithImage:(nullable CGImageRef)imageRef
     uint8_t *rgba = NULL; // RGBA Buffer managed by CFData, don't call `free` on it, instead call `CFRelease` on `dataRef`
     // We must prefer the input CGImage's color space, which may contains ICC profile
     CGColorSpaceRef colorSpace = CGImageGetColorSpace(imageRef);
+    // We only supports RGB colorspace, filter the un-supported one (like Monochrome, CMYK, etc)
+    if (CGColorSpaceGetModel(colorSpace) != kCGColorSpaceModelRGB) {
+        // Ignore and convert, we don't know how to encode this colorspace directlly to WebP
+        // This may cause little visible difference because of colorpsace conversion
+        colorSpace = NULL;
+    }
     if (!colorSpace) {
         colorSpace = [SDImageCoderHelper colorSpaceGetDeviceRGB];
     }