Introduce a generic WebPGetImageReader(type) function

Also introduce an always-failing 'reader' for unknown formats.
So we don't have to check reader==NULL, code is more regular.

-> We can get read of specific ReadPNG(), ReadJPEG(), ... declaration and use.

Change-Id: I290759705420878f00c7223c726d4ad404afd9c4

examples/cwebp.c

@@ -107,7 +107,7 @@ static int ReadPicture(const char* const filename, WebPPicture* const pic,
 
   if (pic->width == 0 || pic->height == 0) {
     WebPImageReader reader = WebPGuessImageReader(data, data_size);
-    ok = (reader != NULL) && reader(data, data_size, pic, keep_alpha, metadata);
+    ok = reader(data, data_size, pic, keep_alpha, metadata);
   } else {
     // If image size is specified, infer it as YUV format.
     ok = ReadYUV(data, data_size, pic);

imageio/image_dec.c

@@ -34,13 +34,28 @@ WebPInputFileFormat WebPGuessImageType(const uint8_t* const data,
   return format;
 }
 
-WebPImageReader WebPGuessImageReader(const uint8_t* const data,
+static int FailReader(const uint8_t* const data, size_t data_size,
-                                     size_t data_size) {
+                      struct WebPPicture* const pic,
-  switch (WebPGuessImageType(data, data_size)) {
+                      int keep_alpha, struct Metadata* const metadata) {
+  (void)data;
+  (void)data_size;
+  (void)pic;
+  (void)keep_alpha;
+  (void)metadata;
+  return 0;
+}
+
+WebPImageReader WebPGetImageReader(WebPInputFileFormat format) {
+  switch (format) {
     case WEBP_PNG_FORMAT: return ReadPNG;
     case WEBP_JPEG_FORMAT: return ReadJPEG;
     case WEBP_TIFF_FORMAT: return ReadTIFF;
     case WEBP_WEBP_FORMAT: return ReadWebP;
-    default: return NULL;
+    default: return FailReader;
   }
-}
+}
+
+WebPImageReader WebPGuessImageReader(const uint8_t* const data,
+                                     size_t data_size) {
+  return WebPGetImageReader(WebPGuessImageType(data, data_size));
+}

imageio/image_dec.h

@@ -49,8 +49,12 @@ typedef int (*WebPImageReader)(const uint8_t* const data, size_t data_size,
                                struct WebPPicture* const pic,
                                int keep_alpha, struct Metadata* const metadata);
 
+// Return the reader associated to a given file format.
+WebPImageReader WebPGetImageReader(WebPInputFileFormat format);
+
 // This function is similar to WebPGuessImageType(), but returns a
-// suitable reader function. Or NULL if the image can't be guessed.
+// suitable reader function. The returned reader is never NULL, but
+// unknown formats will return an always-failing valid reader.
 WebPImageReader WebPGuessImageReader(const uint8_t* const data,
                                      size_t data_size);