[API Proposal]: Implement IEquatable interface on DpiScale

[h3xds1nz]

Background and motivation

Currently, the DpiScale struct does not implement IEquatable, while we have an internal implemented wrapper class DpiScale2 that does implement it and wraps this struct. This is for example used as a dictionary key in StylusLogic, which could then be simplified to use the regular struct and avoid for example temporary heap allocs on lookup.

There were already some boxing issues solved in #6309 but the equality is done with an internal Equals currently, however as Stephen Toub mentions, the hope is for DpiScale to implement IEquatable in the future and I couldn't agree more.

While at it, I'd mark the struct as readonly since it is designed to be immutable anyways.

The hope is to have then 1 unifying type that we can use to represent DpiScale in different scenarios and not having to depend on internal types that are just wrappers around the struct to use it in other places.

API Proposal

namespace System.Windows;

- public partial struct DpiScale
+ public readonly partial struct DpiScale : IEquatable<DpiScale>
{
+   public bool Equals(DpiScale other)
+   public override bool Equals(object obj)
+   public override int GetHashCode()
+   public static bool operator ==(DpiScale left, DpiScale right)
+   public static bool operator !=(DpiScale left, DpiScale right)
}

API Usage

Besides other places:

https://github.com/dotnet/wpf/blob/6c96913d693c56922e697b23b3053aa604d3bd2c/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Common/StylusLogic.cs#L183

Possible additions

We could also add a public utility factory method that's currently also on DpiScale2, that is FromPixelsPerInch, I believe it has its uses: https://github.com/dotnet/wpf/blob/6c96913d693c56922e697b23b3053aa604d3bd2c/src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/DpiScale2.cs#L202

Risks

Adding readonly is not a breaking change, adding IEquatable can obviously produce a new behavior in terms of equality comparisons but I believe it is the right thing to do moving forward.

[miloush]

I wonder why was it worth wrapping it in a new internal type rather than just implementing it directly on the original struct (or creating a comparer).

Anyway, proposal looks good to me.

[h3xds1nz]

I couldn't find anything tangible either even in the old assemblies, it was added in .NetFX 4.8 from what I could dig up; previously from 4.6.2 it was just DpiScale but I guess if they wanted quick and dirty way as there otherwise weren't really public API changes in the latest releases, just bugfixes including Automation stuff.

I was hoping to check some of the hosting scenarios awareness issues and while I knew there are two different members, I didn't know that DpiScale2 was just a plain IEquatable wrapper.