Make structs readonly when applicable (#4002)

* Make all structs readonly when applicable. It should reduce amount of needless defensive copies

* Make structs with trivial boilerplate equality code record structs

* Remove unnecessary readonly modifiers from TextureCreateInfo

* Make BitMap structs readonly too

Ryujinx.Graphics.Texture/Size.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.Graphics.Texture
 {
-    public struct Size
+    public readonly struct Size
     {
         public int Width  { get; }
         public int Height { get; }