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/Translation/Cache/CacheEntry.cs

@@ -4,7 +4,7 @@ using System.Diagnostics.CodeAnalysis;
 
 namespace ARMeilleure.Translation.Cache
 {
-    struct CacheEntry : IComparable<CacheEntry>
+    readonly struct CacheEntry : IComparable<CacheEntry>
     {
         public int Offset { get; }
         public int Size   { get; }