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/AllocationResult.cs

@@ -1,6 +1,6 @@
 namespace ARMeilleure.CodeGen.RegisterAllocators
 {
-    struct AllocationResult
+    readonly struct AllocationResult
     {
         public int IntUsedRegisters { get; }
         public int VecUsedRegisters { get; }