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/Utils/BC7ModeInfo.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.Graphics.Texture.Utils
 {
-    struct BC7ModeInfo
+    readonly struct BC7ModeInfo
     {
         public readonly int SubsetCount;
         public readonly int PartitionBitCount;