Add high-frequency touch events to iOS GameView (MonoGame#8092)

* Fix iOS GamePlatform to run pending background tasks MonoGame#7520

In other `Platform` classes (such as `SDLGamePlatform` and `AndroidGamePlatform`, `Threading.Run()` is called during `Tick()` or `RunLoop()` (inconsistently, it may be called before or after `Game.Tick()` unfortunately).

Fixing this for `iOSGamePlatform`. `WinForms` and perhaps other platforms (Switch?) may suffer from the same issue.

Note that texture loading from background thread is discouraged however a lot of computationally intensive operations (reading/decoding the image file streams, allocating CPU buffers) could be done in the background thread and hence is a good practice to move such work alone into the bgthread until we request the actual texture from the main thread.

* Update docs for sync/test code

* Fix typo in comment

* Add coalesced touch events to iOS GameView

* Add coalesced touch API to TouchPanel*

* Fix UTF-8 copyright mistake

* Ensure we process coalesced touches only if the client asked for it

* Avoid using C#5.0+ features (expression-bodied properties)

* Suggested changes

* Remove Coalesced API (new) and update existing touch event with a flag for `isHighFrequency`.
 - Also consistently use `high-frequency` instead of `coalesced` when referring to this new behavior.

* Remove `out int` C# 7.0 / use C# 5.0 feature instead

* Update based on @harry-cpp comments

* Rename to `origin` instead of `upstream`

* Remove git stuff

* Address PR comments

* Change (c) UTF-8 due to codepage issues, my bad.

---------

Co-authored-by: Tom Spilman <tom@sickheadgames.com>
Co-authored-by: Thao D <thao.mindfulplay@gmail.com>
Co-authored-by: Simon (Darkside) Jackson <darkside@zenithmoon.com>
Co-authored-by: mindfulplay.build <mindfulplay@appu.app>

MonoGame.Framework/Input/Touch/TouchPanel.cs

@@ -1,7 +1,7 @@
 #region License
 // /*
 // Microsoft Public License (Ms-PL)
-// XnaTouch - Copyright © 2009-2010 The XnaTouch Team
+// XnaTouch - Copyright (C) 2009-2010 The XnaTouch Team
 //
 // All rights reserved.
 //