add WebPBlendAlpha() function to blend colors against background

new option: -blend_alpha 0xrrggbb
also: don't force picture.use_argb value for lossless. Instead,
delay the YUVA<->ARGB conversion till WebPEncode() is called.
This make the blending more accurate when source is ARGB
and lossy compression is used (YUVA).
This has an effect on cropping/rescaling. E.g. for PNG, these
are now done in ARGB colorspace instead of YUV when lossy compression
is used.

Change-Id: I18571f1b1179881737a8dbd23ad0aa8cddae3c6b

examples/tiffdec.c

@@ -98,6 +98,7 @@ int ReadTIFF(const char* const filename,
 #ifdef __BIG_ENDIAN__
       TIFFSwabArrayOfLong(raster, width * height);
 #endif
+      pic->use_argb = 1;
       ok = keep_alpha
          ? WebPPictureImportRGBA(pic, (const uint8_t*)raster, stride)
          : WebPPictureImportRGBX(pic, (const uint8_t*)raster, stride);