Avoid POINT allocations

[stephentoub]

Description

There are a bunch of copies around the tree of POINT interop types: some are structs (as they should be), some are classes. The latter are resulting in tons of allocations. This standardizes on using structs for all of them.

In a simple WPF app just moving my mouse around for a few seconds, these are the resulting top allocations:

With this PR, the highlighted line goes away.

Customer Impact

Less allocation, less GC, less pause time.

Regression

No

Testing

CI, plus validation that the allocations went away in the example scenarios I was looking at.

Risk

There is some risk here. The change itself is sound. However, the interop definitions throughout the repo are in a bit of disarray, with many different copies of the same interop types and functions, all defined subtly differently. I believe I caught all of the relevant paths interacting with these types, but it's definitely possible something could have slipped through.

[lindexi]

Can we use the same Point struct type in WPF?

[stephentoub]

Can we use the same Point struct type in WPF?

Do you mean use it for the interop? No, it has a different layout/size, with double rather than int fields.

[lindexi]

@stephentoub

Such as:

wpf/src/Microsoft.DotNet.Wpf/src/Shared/MS/Win32/UnsafeNativeMethodsTextServices.cs

Lines 750 to 766 in 192a38e

	public struct POINT
	{
	/// <summary>
	///
	/// </summary>
	public int x;
	/// <summary>
	///
	/// </summary>
	public int y;
	public POINT(int x, int y)
	{
	this.x = x;
	this.y = y;
	}
	}

wpf/src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClient/MS/Win32/UnsafeNativeMethods.cs

Lines 159 to 169 in 192a38e

	private struct POINT
	{
	public int x;
	public int y;
	public POINT(int x, int y)
	{
	this.x = x;
	this.y = y;
	}
	}

wpf/src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClientSideProviders/MS/Win32/UnsafeNativeMethods.cs

Lines 300 to 310 in 192a38e

	private struct POINT
	{
	public int x;
	public int y;
	public POINT(int x, int y)
	{
	this.x = x;
	this.y = y;
	}
	}

And the POINT in UIAutomation/UIAutomationClient/MS/Win32/UnsafeNativeMethods.cs and src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClientSideProviders/MS/Win32/UnsafeNativeMethods.cs do not add StructLayout attribute.

[stephentoub]

Thanks. This is what I meant by "There are a bunch of copies around the tree of POINT interop types"; in fact, tons of interop is duplicated around the tree. It could certainly be cleaned up / deduped / etc., but I'm not going to do that here.