Add posibility to set a webp decoding function

Now a drag&drop operation in Windows can use this custom webp decoding
function to decode the first frame of an element dropped as webp data.

os/dnd.cpp

@@ -19,6 +19,7 @@ static DecoderFunc g_decode_png = default_decode_png;
 static DecoderFunc g_decode_jpg = default_decode_jpg;
 static DecoderFunc g_decode_bmp = default_decode_bmp;
 static DecoderFunc g_decode_gif = default_decode_gif;
+static DecoderFunc g_decode_webp = nullptr;
 
 void set_decode_png(DecoderFunc func)
 {
@@ -40,6 +41,11 @@ void set_decode_gif(DecoderFunc func)
   g_decode_gif = func;
 }
 
+void set_decode_webp(DecoderFunc func)
+{
+  g_decode_webp = func;
+}
+
 SurfaceRef decode_png(const uint8_t* buf, const uint32_t len)
 {
   return g_decode_png(buf, len);
@@ -60,6 +66,11 @@ SurfaceRef decode_gif(const uint8_t* buf, const uint32_t len)
   return g_decode_gif(buf, len);
 }
 
+SurfaceRef decode_webp(const uint8_t* buf, const uint32_t len)
+{
+  return g_decode_webp ? g_decode_webp(buf, len) : nullptr;
+}
+
 } // namespace os
 
 #endif

os/dnd.h

@@ -36,11 +36,13 @@ namespace os {
   void set_decode_jpg(DecoderFunc func);
   void set_decode_bmp(DecoderFunc func);
   void set_decode_gif(DecoderFunc func);
+  void set_decode_webp(DecoderFunc func);
 
   SurfaceRef decode_png(const uint8_t* buf, uint32_t len);
   SurfaceRef decode_jpg(const uint8_t* buf, uint32_t len);
   SurfaceRef decode_bmp(const uint8_t* buf, uint32_t len);
   SurfaceRef decode_gif(const uint8_t* buf, uint32_t len);
+  SurfaceRef decode_webp(const uint8_t* buf, uint32_t len);
 #endif
   // Operations that can be supported by source and target windows in a drag
   // and drop operation.

os/win/dnd.cpp

@@ -240,6 +240,9 @@ SurfaceRef DragDataProviderWin::getImage()
 
         if (ext == "BMP")
           return os::decode_bmp(content, content.size());
+
+        if (ext == "WEBP")
+          return os::decode_webp(content, content.size());
       }
     }
   }
@@ -303,7 +306,8 @@ bool DragDataProviderWin::contains(DragDataItemType type)
                 std::string ext = base::get_file_extension(filename);
                 std::transform(ext.begin(), ext.end(), ext.begin(), ::toupper);
                 if (ext == "PNG" || ext == "JPG" || ext == "JPEG" ||
-                    ext == "JPE" || ext == "GIF" || ext == "BMP")
+                    ext == "JPE" || ext == "GIF" || ext == "BMP" ||
+                    ext == "WEBP")
                   return true;
               }
             }