Blitting from the MainFrame's own wxClientDC clips out child HWNDs, so all of
OrcaSlicer's custom child-window controls (sidebar buttons/combos/panels) and the
GL canvas came back as uninitialized black bitmap memory. Read the window's
on-screen rectangle from the DWM-composited desktop via wxScreenDC instead, which
includes every child window, the OpenGL surface, and ImGui overlays.
Document the visible/unobscured requirement and the HiDPI logical-vs-physical
pixel caveat; clarify how screenshot.viewport3d differs and why it stays.
