Fixes pixelSpecificationNarrower for the png plugin.

Summary: In some cases (e.g when the input image would have a `skipFirst` / `skipLast` alphaInfo) encoding an image to png would fail as the narrowed pixel spec wasn't accepted by the png compressor.

Reviewed By: lambdapioneer

Differential Revision: D13693679

fbshipit-source-id: c8793147a468e676aaa5570d1e148b903cfe6075

cpp/spectrum/plugins/png/LibPngTranscodingPlugin.cpp

@@ -30,11 +30,17 @@ inline codecs::DecompressorProvider::Factory makeLibPngDecompressorFactory() {
 
 image::pixel::Specification pixelSpecificationNarrower(
     const image::pixel::Specification& pixelSpecification) {
-  if (pixelSpecification == image::pixel::specifications::RGBA) {
-    // PNGs cannot encode RGBA.
-    return image::pixel::specifications::ARGB;
+  if (pixelSpecification.colorModel == image::pixel::colormodels::Gray) {
+    return image::pixel::specifications::Gray;
   } else {
-    return pixelSpecification;
+    switch (pixelSpecification.alphaInfo) {
+      case image::pixel::AlphaInfo::None:
+      case image::pixel::AlphaInfo::SkipLast:
+      case image::pixel::AlphaInfo::SkipFirst:
+        return image::pixel::specifications::RGB;
+      default:
+        return image::pixel::specifications::ARGB;
+    }
   }
 }