Add alphaType() to SkImage

Keep isOpaque as a convenience method -- many places really only need to
know that for optimization purposes (SrcOver -> Src, etc...).

In all the places where we pull data back out or convert to another
object and need to supply an SkImageInfo, we can avoid losing information
about premulness.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2250663002

Review-Url: https://codereview.chromium.org/2250663002
diff --git a/gm/image.cpp b/gm/image.cpp
index a67ed20..400d421 100644
--- a/gm/image.cpp
+++ b/gm/image.cpp
@@ -303,8 +303,7 @@
 #include "SkImageGenerator.h"
 
 static SkImageInfo make_info(SkImage* img) {
-    return SkImageInfo::MakeN32(img->width(), img->height(),
-                                img->isOpaque() ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
+    return SkImageInfo::MakeN32(img->width(), img->height(), img->alphaType());
 }
 
 // Its simple, but I wonder if we should expose this formally?