introduce a generic WebPPictureHasTransparency() function

VP8-lossy will now avoid writing an ALPH chunk if the
alpha values are trivial.

+ changed DumpPicture() accordingly in cwebp
+ prevented the -d option to be active with lossless
 (DumpPicture wouldn't work).

Change-Id: I34fdb108a2b6207e93fa6cd00b1d2509a8e1dc4b

src/enc/alpha.c

@@ -281,7 +281,7 @@ static int EncodeAlpha(const uint8_t* data, int width, int height, int stride,
 // Main calls
 
 void VP8EncInitAlpha(VP8Encoder* enc) {
-  enc->has_alpha_ = (enc->pic_->a != NULL);
+  enc->has_alpha_ = WebPPictureHasTransparency(enc->pic_);
   enc->alpha_data_ = NULL;
   enc->alpha_data_size_ = 0;
 }