Capture with ImGuiCaptureFlags_StitchAll fails with multi-viewports

Re-Bonjour Omar,

Sorry if I'm flooding the issues a bit today.

I'm trying the code of app_minimal, and I have an issue with screen capture and the flag ImGuiCaptureFlags_StitchAll.

Basically,

• I'm getting a fully black capture under windows (although it has the correct size). This is strange. I'm using Windows 10 ARM 64 bits (although I compiled the code in x64 mode)
• under macOS (after having applied a hack to take into account the FrameBufferScale), the scroll does not work, and I get a repeated image:

i.e. the size is correct, but it seems like the content is repeated (as if the scroll did not work).
Based on the usleep(1000) for linux, I tried to add a sleep(1 second) during the capture, and I could see that effectively the scroll does not work.

Start with the app_minimal capture path using ImGuiCaptureFlags_StitchAll and reproduce the report on Windows 10 ARM64/x64 and macOS. Investigate the multi-viewport capture behavior, including scrolling and framebuffer scaling; done means the capture is not black or repeated and its contents reflect the scrolled view.