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/OpCodeTable.cs

@@ -11,7 +11,7 @@ namespace ARMeilleure.Decoders
 
         private const int FastLookupSize = 0x1000;
 
-        private struct InstInfo
+        private readonly struct InstInfo
         {
             public int Mask  { get; }
             public int Value { get; }