ImGui.NET for MonoGame

Fork of https://github.com/mellinoe/ImGui.NET with the following changes.

• Ported ImGui.Net to DotNet Core 3.1.
• Replaced types from System.Numerics in ImGui.Net with similar types from MonoGame 3.8
• Ported MonoGame example to DotNet Core 3.1.
• Upgraded example to MonoGame 3.8.
• Fixed a bug in the MonoGame example that led everything to be offset with 0.5 pixel, causing unsharp text.