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.Vulkan/StagingBuffer.cs

@@ -14,7 +14,7 @@ namespace Ryujinx.Graphics.Vulkan
         private readonly VulkanRenderer _gd;
         private readonly BufferHolder _buffer;
 
-        private struct PendingCopy
+        private readonly struct PendingCopy
         {
             public FenceHolder Fence { get; }
             public int Size { get; }