Add predictive filtering option for Alpha.

Add predictive filtering option for Alpha plane.
Valid range for filter option is [0, 5] corresponding to prediction
methods none, horizontal, vertical, gradient & paeth filter.
The prediction method 5 will try all the prediction methods (0 to 4)
and pick the prediction method that gives best compression.

Change-Id: I9244d4a9c5017501a9696c7cec5045f04c16d49b

src/utils/filters.h

@@ -0,0 +1,50 @@
+// Copyright 2011 Google Inc.
+//
+// This code is licensed under the same terms as WebM:
+//  Software License Agreement:  http://www.webmproject.org/license/software/
+//  Additional IP Rights Grant:  http://www.webmproject.org/license/additional/
+// -----------------------------------------------------------------------------
+//
+// Spatial prediction using various filters
+//
+// Author: Urvang (urvang@google.com)
+
+#ifndef WEBP_UTILS_FILTERS_H_
+#define WEBP_UTILS_FILTERS_H_
+
+#include "../webp/types.h"
+
+#if defined(__cplusplus) || defined(c_plusplus)
+extern "C" {
+#endif
+
+// Filters.
+typedef enum {
+  WEBP_FILTER_NONE = 0,
+  WEBP_FILTER_HORIZONTAL,
+  WEBP_FILTER_VERTICAL,
+  WEBP_FILTER_GRADIENT,
+  WEBP_FILTER_PAETH,
+  WEBP_FILTER_BEST,
+  WEBP_FILTER_LAST,
+} WEBP_FILTER_TYPE;
+
+typedef void (*WebPFilterFunc)(const uint8_t* in, int width, int height,
+                               int bpp, int stride, uint8_t* out);
+
+// Filter the given data using the given predictor.
+// 'in' corresponds to a 2-dimensional pixel array of size (stride * height)
+// in raster order.
+// 'bpp' is number of bytes per pixel, and
+// 'stride' is number of bytes per scan line (with possible padding).
+// 'out' should be pre-allocated.
+extern const WebPFilterFunc WebPFilters[/*WEBP_FILTER_LAST*/];
+
+// Reconstruct the original data from the given filtered data.
+extern const WebPFilterFunc WebPUnfilters[/*WEBP_FILTER_LAST*/];
+
+#if defined(__cplusplus) || defined(c_plusplus)
+}    // extern "C"
+#endif
+
+#endif  /* WEBP_UTILS_FILTERS_H_ */