only apply color-mapping for alpha on the cropped area

This is only possible if the filtering is not VERTICAL or GRADIENT.
Otherwise, we need the spatial predictors and hence need the un-visible
part above crop_top row.

COLOR_INDEX transform is the only transform that is not predicted
from previous row. Applying the same for other transform (spatial
predict, ...) is going to be more involve and use an extra temporary row.

+ remove ApplyInverseTransformsAlpha()
(work is done directly within ExtractPalettedAlphaRows())

+ change back to using filter_ instead of unfilter_func_

Change-Id: I09e57efae4a4af00bde35f21ca6e3d73b35d7d43

src/dec/alphai.h

@@ -28,7 +28,7 @@ struct ALPHDecoder {
   int width_;
   int height_;
   int method_;
-  WebPUnfilterFunc unfilter_func_;
+  WEBP_FILTER_TYPE filter_;
   int pre_processing_;
   struct VP8LDecoder* vp8l_dec_;
   VP8Io io_;