Some Linux/Mac players get libgdiplus DLL-not-found error when mod loads PNG

[Pathoschild]

When the content API added by #257 is used to load a PNG, some Linux/Mac users get this error:

A mod failed handling the ControlEvents.KeyPressed event:
Microsoft.Xna.Framework.Content.ContentLoadException: Stardewponics failed loading content asset 'assets/greenhouse.png' from ModFolder.
---> System.InvalidOperationException: This image format is not supported
---> System.TypeInitializationException: The type initializer for 'System.Drawing.GDIPlus' threw an exception.
---> System.DllNotFoundException: libgdiplus.dylib

This isn't unique to SMAPI — many players have reported the same problem with the GetDressed mod on Linux/Mac. The problem happens because the libgdiplus library isn't included in the game's local version of Mono, though it doesn't affect all players.

[Pathoschild]

Based on testing with one affected player, copying libgdiplus.dylib into the game folder seems to work. We'll need to see if the same file works for others; if so, we can just add it to the install package for Linux/Mac players.

[Pathoschild]

Done in develop for the next SMAPI release.