mainthread: support windows

[gucio321]

We use mainthread libraries to allow giu commands on any thread (when imgui requires to run on main)

We currently have implementations for macos and linux but for windows none works.

• ref: [bug] An error occurs when you run the example code for the master branch on Windows 11 pro #735
• also: proposal: runtime: add LockMainOSThread to take control of the main thread golang/go#64755

[gucio321]

hmm, well, when I look at it @AllenDang why are we using this miainthread in fact?
Am I right above?

[AllenDang]

As I remember, OpenGL requires to run GPU related operations in the main thread, so I use mainthread to archive that.
And at that time, I'm using golang wrapper of glfw to write a backend manually, which works great on linux/Mac/windows, for now, we are using glfw backend directly from Imgui right? Maybe we don't need mainthread anymore? Like the backend example in cimgui shows.

[gucio321]

so in fact we just need to call NewMasterWindow in the same thread as Run, right?