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.Shader/Decoders/Block.cs

@@ -17,7 +17,7 @@ namespace Ryujinx.Graphics.Shader.Decoders
         }
     }
 
-    struct SyncTarget
+    readonly struct SyncTarget
     {
         public PushOpInfo PushOpInfo { get; }
         public int PushOpId { get; }

Ryujinx.Graphics.Shader/Decoders/DecodedProgram.cs

@@ -4,7 +4,7 @@ using System.Collections.Generic;
 
 namespace Ryujinx.Graphics.Shader.Decoders
 {
-    struct DecodedProgram : IEnumerable<DecodedFunction>
+    readonly struct DecodedProgram : IEnumerable<DecodedFunction>
     {
         public DecodedFunction MainFunction { get; }
         private readonly IReadOnlyDictionary<ulong, DecodedFunction> _functions;

Ryujinx.Graphics.Shader/Decoders/Decoder.cs

@@ -473,7 +473,7 @@ namespace Ryujinx.Graphics.Shader.Decoders
             op = Unsafe.As<ulong, T>(ref rawOp);
         }
 
-        private struct BlockLocation
+        private readonly struct BlockLocation
         {
             public Block Block { get; }
             public int Index { get; }

Ryujinx.Graphics.Shader/Decoders/InstTable.cs

@@ -6,7 +6,7 @@ namespace Ryujinx.Graphics.Shader.Decoders
     {
         private const int EncodingBits = 14;
 
-        private struct TableEntry
+        private readonly struct TableEntry
         {
             public InstName Name { get; }
             public InstEmitter Emitter { get; }

Ryujinx.Graphics.Shader/Decoders/Register.cs

@@ -2,7 +2,7 @@ using System;
 
 namespace Ryujinx.Graphics.Shader.Decoders
 {
-    struct Register : IEquatable<Register>
+    readonly struct Register : IEquatable<Register>
     {
         public int Index { get; }