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/CodeGen/RegisterAllocators/CopyResolver.cs

@@ -11,7 +11,7 @@ namespace ARMeilleure.CodeGen.RegisterAllocators
     {
         private class ParallelCopy
         {
-            private struct Copy
+            private readonly struct Copy
             {
                 public Register Dest   { get; }
                 public Register Source { get; }