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/webp/decode.h

@@ -113,12 +113,35 @@ typedef enum { MODE_RGB = 0, MODE_RGBA = 1,
                MODE_BGR = 2, MODE_BGRA = 3,
                MODE_ARGB = 4, MODE_RGBA_4444 = 5,
                MODE_RGB_565 = 6,
+               // RGB-premultiplied transparent modes (alpha value is preserved)
+               MODE_rgbA = 7,
+               MODE_bgrA = 8,
+               MODE_Argb = 9,
+               MODE_rgbA_4444 = 10,
                // YUV modes must come after RGB ones.
-               MODE_YUV = 7, MODE_YUVA = 8,  // yuv 4:2:0
-               MODE_LAST = 9
+               MODE_YUV = 11, MODE_YUVA = 12,  // yuv 4:2:0
+               MODE_LAST = 13
              } WEBP_CSP_MODE;
 
-// Generic structure for describing the sample buffer.
+// Some useful macros:
+static WEBP_INLINE int WebPIsPremultipliedMode(WEBP_CSP_MODE mode) {
+  return (mode == MODE_rgbA || mode == MODE_bgrA || mode == MODE_Argb ||
+          mode == MODE_rgbA_4444);
+}
+
+static WEBP_INLINE int WebPIsAlphaMode(WEBP_CSP_MODE mode) {
+  return (mode == MODE_RGBA || mode == MODE_BGRA || mode == MODE_ARGB ||
+          mode == MODE_RGBA_4444 || mode == MODE_YUVA ||
+          WebPIsPremultipliedMode(mode));
+}
+
+static WEBP_INLINE int WebPIsRGBMode(WEBP_CSP_MODE mode) {
+  return (mode < MODE_YUV);
+}
+
+//------------------------------------------------------------------------------
+// WebPDecBuffer: Generic structure for describing the sample buffer.
+
 typedef struct {    // view as RGBA
   uint8_t* rgba;    // pointer to RGBA samples
   int stride;       // stride in bytes from one scanline to the next.

src/webp/decode_vp8.h

@@ -99,7 +99,9 @@ struct VP8Io {
   int use_scaling;
   int scaled_width, scaled_height;
 
-  // pointer to the alpha data (if present) corresponding to the rows
+  // If non NULL, pointer to the alpha data (if present) corresponding to the
+  // start of the current row (That is: it is pre-offset by mb_y and takes
+  // cropping into account).
   const uint8_t* a;
 };