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.Texture/Region.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.Graphics.Texture
 {
-    public struct Region
+    public readonly struct Region
     {
         public int Offset { get; }
         public int Size { get; }

Ryujinx.Graphics.Texture/Size.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.Graphics.Texture
 {
-    public struct Size
+    public readonly struct Size
     {
         public int Width  { get; }
         public int Height { get; }

Ryujinx.Graphics.Texture/SizeInfo.cs

@@ -3,7 +3,7 @@ using System.Collections.Generic;
 
 namespace Ryujinx.Graphics.Texture
 {
-    public struct SizeInfo
+    public readonly struct SizeInfo
     {
         private readonly int[] _mipOffsets;
 

Ryujinx.Graphics.Texture/Utils/BC7ModeInfo.cs

@@ -1,6 +1,6 @@
 namespace Ryujinx.Graphics.Texture.Utils
 {
-    struct BC7ModeInfo
+    readonly struct BC7ModeInfo
     {
         public readonly int SubsetCount;
         public readonly int PartitionBitCount;