add colorspace for premultiplied alpha

The new modes are
       MODE_rgbA
       MODE_bgrA
       MODE_Argb
       MODE_rgbA_4444
It's binary incompatible, since the enums changed.

While at it, i removed the now unneeded KeepAlpha methods.
-> Saved ~12k of code!

* made explicit mention that alpha_plane is persistent,
so we have access to the full alpha plane data at all time.
Incremental decoding of alpha was planned for, but not
implemented. So better not dragged this constaint for now
and make the code easier until we revisit that.

Change-Id: Idaba281a6ca819965ca062d1c23329f36d90c7ff

src/dec/webpi.h

@@ -24,6 +24,7 @@ extern "C" {
 
 typedef struct WebPDecParams WebPDecParams;
 typedef int (*OutputFunc)(const VP8Io* const io, WebPDecParams* const p);
+typedef int (*OutputRowFunc)(WebPDecParams* const p, int y_pos);
 
 struct WebPDecParams {
   WebPDecBuffer* output;             // output buffer.
@@ -34,9 +35,11 @@ struct WebPDecParams {
   const WebPDecoderOptions* options;  // if not NULL, use alt decoding features
   // rescalers
   WebPRescaler scaler_y, scaler_u, scaler_v, scaler_a;
-  void* memory;               // overall scratch memory for the output work.
-  OutputFunc emit;            // output RGB or YUV samples
-  OutputFunc emit_alpha;      // output alpha channel
+  void* memory;                  // overall scratch memory for the output work.
+
+  OutputFunc emit;               // output RGB or YUV samples
+  OutputFunc emit_alpha;         // output alpha channel
+  OutputRowFunc emit_alpha_row;  // output one line of rescaled alpha values
 };
 
 // Should be called first, before any use of the WebPDecParams object.