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/ShaderSource.cs

@@ -3,7 +3,7 @@ using Ryujinx.Graphics.Shader.Translation;
 
 namespace Ryujinx.Graphics.GAL
 {
-    public struct ShaderSource
+    public readonly struct ShaderSource
     {
         public string Code { get; }
         public byte[] BinaryCode { get; }