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.GAL/Rectangle.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.Graphics.GAL
 {
-    public struct Rectangle<T> where T : unmanaged
+    public readonly struct Rectangle<T> where T : unmanaged
     {
         public T X { get; }
         public T Y { get; }