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/Diagnostics/Symbols.cs

@@ -6,7 +6,7 @@ namespace ARMeilleure.Diagnostics
 {
     static class Symbols
     {
-        private struct RangedSymbol
+        private readonly struct RangedSymbol
         {
             public readonly ulong Start;
             public readonly ulong End;