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.Gpu/Shader/GpuChannelPoolState.cs

@@ -5,7 +5,7 @@ namespace Ryujinx.Graphics.Gpu.Shader
     /// <summary>
     /// State used by the <see cref="GpuAccessor"/>.
     /// </summary>
-    struct GpuChannelPoolState : IEquatable<GpuChannelPoolState>
+    readonly struct GpuChannelPoolState : IEquatable<GpuChannelPoolState>
     {
         /// <summary>
         /// GPU virtual address of the texture pool.