fix transparency issues on Linux/Mac due to MonoGame bug (#279)

docs/release-notes.md

@@ -10,6 +10,7 @@
   * Fixed crash when a mod manifest is corrupted.
   * Fixed error-handling when initialising paths.
   * Fixed 'no update keys' warning not shown for mods with only invalid update keys.
+  * Fixed transparency issues on Linux/Mac for many mods.
   * Updated compatibility list.
 
 * For modders:

src/SMAPI/Framework/ContentManagers/ModContentManager.cs

@@ -171,6 +171,14 @@ 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);
             for (int i = 0; i < data.Length; i++)