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.Graphics.Nvdec/FrameDecodedEventArgs.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.Graphics.Nvdec
 {
-    public struct FrameDecodedEventArgs
+    public readonly struct FrameDecodedEventArgs
     {
         public CodecId CodecId { get; }
         public uint LumaOffset { get; }

Ryujinx.Graphics.Nvdec/ResourceManager.cs

@@ -3,7 +3,7 @@ using Ryujinx.Graphics.Nvdec.Image;
 
 namespace Ryujinx.Graphics.Nvdec
 {
-    struct ResourceManager
+    readonly struct ResourceManager
     {
         public MemoryManager Gmm { get; }
         public SurfaceCache Cache { get; }