remove tcoder, switch alpha-plane compression to lossless

* Method #1 is now calling the lossless encoder on the alpha plane.
Format is not final, it's just a first draft. We need ad-hoc functions.
* removed now useless utils/alpha.*
* added utils/quant_levels.h instead
* removed the TCoder code altogether

Change-Id: I636840b6129a43171b74860e0a0fc5bb1bcffc6a

src/utils/quant_levels.h

@@ -0,0 +1,34 @@
+// Copyright 2011 Google Inc. All Rights Reserved.
+//
+// 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/
+// -----------------------------------------------------------------------------
+//
+// Alpha plane quantization utility
+//
+// Author: vikasa@google.com (Vikas Arora)
+
+#ifndef WEBP_UTILS_QUANT_LEVELS_H_
+#define WEBP_UTILS_QUANT_LEVELS_H_
+
+#include <stdlib.h>
+
+#include "../webp/types.h"
+
+#if defined(__cplusplus) || defined(c_plusplus)
+extern "C" {
+#endif
+
+// Replace the input 'data' of size 'width'x'height' with 'num-levels'
+// quantized values. If not NULL, 'mse' will contain the mean-squared error.
+// Valid range for 'num_levels' is [2, 256].
+// Returns false in case of error (data is NULL, or parameters are invalid).
+int QuantizeLevels(uint8_t* data, int width, int height, int num_levels,
+                   float* mse);
+
+#if defined(__cplusplus) || defined(c_plusplus)
+}    // extern "C"
+#endif
+
+#endif  /* WEBP_UTILS_QUANT_LEVELS_H_ */