Implement SkGifCodec::onSkipScanlines()

src/codec/SkCodec_libgif.h

Index: src/codec/SkCodec_libgif.h
diff --git a/src/codec/SkCodec_libgif.h b/src/codec/SkCodec_libgif.h
index 31e0139b08ae82304b213ea02ec10a138b59d4f8..31a1d8ea4aec85879242a2f39673bd4336e67dcb 100644
--- a/src/codec/SkCodec_libgif.h
+++ b/src/codec/SkCodec_libgif.h
@@ -150,6 +150,25 @@ private:
 
     int onGetScanlines(void* dst, int count, size_t rowBytes) override;
 
+    bool onSkipScanlines(int count) override;
+
+    /*
+     * For a scanline decode of "count" lines, this function indicates how
+     * many of the "count" lines should be skipped until we reach the top of
+     * the image frame and how many of the "count" lines are acutally inside

[scroggo, 2015/11/19 22:23:06]

actually*

[msarett, 2015/11/19 22:39:22]

Done.

+     * the image frame.
+     *
+     * This only needs to be called when the image frame being decoded is
+     * a subset of the size of the entire image.

[scroggo, 2015/11/19 22:23:06]

i.e. fFrameIsSubset

Alternatively, this method could do the following:

if (fFrameIsSubset) {
    // Calculate the output vars
} else {
    // Set them to 0 and count
}

[msarett, 2015/11/19 22:39:22]

Yeah, let's do it this way.

+     *
+     * @param count           The number of scanlines requested.
+     * @param rowsBeforeFrame The number of lines before we reach the top of

[scroggo, 2015/11/19 22:23:06]

Maybe specify that these are output variables?

[msarett, 2015/11/19 22:39:22]

Done.

+     *                        the image frame.
+     * @param rowsInFrame     The number of lines to decode inside the image
+     *                        frame.
+     */
+    void handleScanlineFrame(int count, int* rowsBeforeFrame, int* rowsInFrame);
+
     SkScanlineOrder onGetScanlineOrder() const override;
 
     /*