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/RegisterMasks.cs

@@ -3,7 +3,7 @@ using System;
 
 namespace ARMeilleure.CodeGen.RegisterAllocators
 {
-    struct RegisterMasks
+    readonly struct RegisterMasks
     {
         public int IntAvailableRegisters   { get; }
         public int VecAvailableRegisters   { get; }