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

ARMeilleure/Decoders/InstDescriptor.cs

@@ -2,7 +2,7 @@ using ARMeilleure.Instructions;
 
 namespace ARMeilleure.Decoders
 {
-    struct InstDescriptor
+    readonly struct InstDescriptor
     {
         public static InstDescriptor Undefined => new InstDescriptor(InstName.Und, InstEmit.Und);