fix semi-transparency issues on Linux/Mac

Apparently Mono no longer premultiplies loaded PNGs by default.

docs/release-notes.md

@@ -3,6 +3,7 @@
 # Release notes
 ## Upcoming release
 * For players:
+  * Fixed semi-transparency issues on Linux/Mac in recent versions of Mono (e.g. pink shadows).
   * Updated translations. Thanks to Annosz (added Hungarian)!
 
 * For modders:

src/SMAPI/Framework/ContentManagers/ModContentManager.cs

@@ -241,13 +241,6 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <remarks>Based on <a href="https://gamedev.stackexchange.com/a/26037">code by David Gouveia</a>.</remarks>
         private Texture2D PremultiplyTransparency(Texture2D texture)
         {
-            // Textures loaded by Texture2D.FromStream are already premultiplied on Linux/Mac, even
-            // though the XNA documentation explicitly says otherwise. That's a glitch in MonoGame
-            // fixed in newer versions, but the game uses a bundled version that will always be
-            // affected. See https://github.com/MonoGame/MonoGame/issues/4820 for more info.
-            if (Constants.TargetPlatform != GamePlatform.Windows)
-                return texture;
-
             // premultiply pixels
             Color[] data = new Color[texture.Width * texture.Height];
             texture.GetData(data);