Fixes SKFontStyleSlant to FontStyle conversion

src/Avalonia.Visuals/Media/FontStyle.cs

@@ -11,13 +11,13 @@ namespace Avalonia.Media
         Normal,
 
         /// <summary>
-        /// An oblique font.
+        /// An italic font.
         /// </summary>
-        Oblique,
+        Italic,
 
         /// <summary>
-        /// An italic font.
+        /// An oblique font.
         /// </summary>
-        Italic,
+        Oblique
     }
 }

src/Skia/Avalonia.Skia/SKTypefaceCollectionCache.cs

@@ -56,7 +56,8 @@ namespace Avalonia.Skia
                     continue;
                 }
 
-                var key = new FontKey(fontFamily.Name, (FontStyle)typeface.FontSlant, (FontWeight)typeface.FontWeight);
+                var key = new FontKey(fontFamily.Name, typeface.FontSlant.ToAvalonia(),
+                    (FontWeight)typeface.FontWeight);
 
                 typeFaceCollection.AddTypeface(key, typeface);
             }

src/Skia/Avalonia.Skia/SkiaSharpExtensions.cs

@@ -138,6 +138,17 @@ namespace Avalonia.Skia
             }
         }
 
+        public static FontStyle ToAvalonia(this SKFontStyleSlant slant)
+        {
+            return slant switch
+            {
+                SKFontStyleSlant.Upright => FontStyle.Normal,
+                SKFontStyleSlant.Italic => FontStyle.Italic,
+                SKFontStyleSlant.Oblique => FontStyle.Oblique,
+                _ => throw new ArgumentOutOfRangeException(nameof (slant), slant, null)
+            };
+        }
+
         public static SKPath Clone(this SKPath src)
         {
             return src != null ? new SKPath(src) : null;