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

Ryujinx.HLE/Ui/ThemeColor.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.HLE.Ui
 {
-    public struct ThemeColor
+    public readonly struct ThemeColor
     {
         public float A { get; }
         public float R { get; }