Disable png encodes from Alpha8, Float16

src/images/SkPNGImageEncoder.cpp

Index: src/images/SkPNGImageEncoder.cpp
diff --git a/src/images/SkPNGImageEncoder.cpp b/src/images/SkPNGImageEncoder.cpp
index 1c363edf04dad3443127a71c95838a0b245a412b..9b8808a2e0e765fbccfaabd40f1c3a9a12673718 100644
--- a/src/images/SkPNGImageEncoder.cpp
+++ b/src/images/SkPNGImageEncoder.cpp
@@ -195,10 +195,7 @@ bool SkPNGImageEncoder::onEncode(SkWStream* stream,
         case kRGB_565_SkColorType:
             break;
         default:
-            // TODO(scroggo): support Alpha_8 as Grayscale(black)+Alpha
-            if (originalBitmap.copyTo(&copy, kN32_SkColorType)) {
-                bitmap = ©
-            }
+            return false;
     }
     SkColorType ct = bitmap->colorType();