LibGfx: Add first_animated_frame_index method to ImageDecoder

Some image formats such as APNG may not use the first frame for
animations.

Userland/Libraries/LibGfx/ImageFormats/QOILoader.h

@@ -50,6 +50,7 @@ public:
     virtual bool is_animated() override { return false; }
     virtual size_t loop_count() override { return 0; }
     virtual size_t frame_count() override { return 1; }
+    virtual size_t first_animated_frame_index() override { return 0; }
     virtual ErrorOr<ImageFrameDescriptor> frame(size_t index) override;
     virtual ErrorOr<Optional<ReadonlyBytes>> icc_data() override;