Implement a software ETC2 texture decoder (#4121)

* Implement a software ETC2 texture decoder

* Fix output size calculation for non-2D textures

* Address PR feedback

Ryujinx.Graphics.GAL/Format.cs

@@ -516,6 +516,27 @@ namespace Ryujinx.Graphics.GAL
             return false;
         }
 
+        /// <summary>
+        /// Checks if the texture format is an ETC2 format.
+        /// </summary>
+        /// <param name="format">Texture format</param>
+        /// <returns>True if the texture format is an ETC2 format, false otherwise</returns>
+        public static bool IsEtc2(this Format format)
+        {
+            switch (format)
+            {
+                case Format.Etc2RgbaSrgb:
+                case Format.Etc2RgbaUnorm:
+                case Format.Etc2RgbPtaSrgb:
+                case Format.Etc2RgbPtaUnorm:
+                case Format.Etc2RgbSrgb:
+                case Format.Etc2RgbUnorm:
+                    return true;
+            }
+
+            return false;
+        }
+
         /// <summary>
         /// Checks if the texture format is a BGR format.
         /// </summary>