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.Ava/Ui/Windows/IconColorPicker.cs

@@ -20,7 +20,7 @@ namespace Ryujinx.Ava.Ui.Windows
 
         private const int CutOffLuminosity = 64;
 
-        private struct PaletteColor
+        private readonly struct PaletteColor
         {
             public int Qck { get; }
             public byte R { get; }