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.Shader/Translation/ShaderHeader.cs

@@ -12,7 +12,7 @@ namespace Ryujinx.Graphics.Shader.Translation
         ScreenLinear = 3
     }
 
-    struct ImapPixelType
+    readonly struct ImapPixelType
     {
         public PixelImap X { get; }
         public PixelImap Y { get; }