Add currScanline() getter to SkCodec API

src/codec/SkCodec_libgif.cpp

Index: src/codec/SkCodec_libgif.cpp
diff --git a/src/codec/SkCodec_libgif.cpp b/src/codec/SkCodec_libgif.cpp
index 8021af95a2e050f1b2df82985d9f467203e108b2..e6a40164f3549e591a569ba364a7eb78275c95d7 100644
--- a/src/codec/SkCodec_libgif.cpp
+++ b/src/codec/SkCodec_libgif.cpp
@@ -508,7 +508,7 @@ SkCodec::Result SkGifCodec::onStartScanlineDecode(const SkImageInfo& dstInfo,
 
 void SkGifCodec::handleScanlineFrame(int count, int* rowsBeforeFrame, int* rowsInFrame) {
     if (fFrameIsSubset) {
-        const int currRow = this->INHERITED::nextScanline();

[msarett, 2015/12/01 23:23:04]

Technically this works...

What I intended was for this call to return fCurrScanline.  What it actually
does is call SkGifCodec::onOutputScanline(fCurrScanline).

For kTopDown this works because SkGifCodec::onOutputScanline(fCurrScanline)
returns fCurrScanline.

For kOutOfOrder (interlaced) this works because we only really need to know
whether the current row is inside or outside of the image frame. 
SkGifCodec::onOutputScanline(fCurrScanline) gets this part right at least.  But
it also makes an unnecessary call to get_output_row_interlaced which will give
us a strange location within the image frame (this doesn't cause a problem
because we only ever get or skip one row at a time for kOutOfOrder images).

Regardless, it's simpler and more efficient to just call currScanline().

[scroggo, 2015/12/02 15:24:41]

The old code is also weird because SkCodec::nextScanline is not virtual (maybe
it used to be, or we were calling another function, and the INHERITED:: stuck
around).

So, IIUC, for an interlaced subset frame gif, the old code returns the wrong
number, but this doesn't matter, because it correctly returns a number inside
the frame?

[msarett, 2015/12/02 15:45:05]

Yes that is correct.

+        const int currRow = this->currScanline();
 
         // The number of rows that remain to be skipped before reaching rows that we
         // actually must decode into.