[3.0] CursorGrabbed doesn't work when using the SDL backend.

[NogginBops]

Description

Setting CursorGrabbed = true and using Mouse.GetState() will return mouse coordinates that are locked within the window instead of working as expected (virtual coordinates).

Repro steps

Please provide the steps required to reproduce the problem

1. Set CursorGrabbed = true.
2. Read mouse position from Mouse.GetState() while moving the mouse towards the edge of the window.
3. See that the mouse coordinates get stuck when the edge of the window is reached.

Expected behavior

For the coordinates to keep increasing past the edge.

Actual behavior

The coordinates get stuck.

Related information

• Operating system: Windows
• SDL or Native backend (if using OpenTK 3.x): SDL
• What version of OpenTK (Installer, NuGet, Source Code): 3.3.3
• What runtime (.NET Framework, CoreCLR, Mono): .net framework 4.8
• Workarounds (if any): Don't use SDL backend