Allow to also blit back the depth buffer from the internal framebuffer
In an OpenGL application that we recently ported to VTK 9.3.0 (www.MeVisLab.de), we use vtkExternalOpenGLRenderWindow to render into an existing OpenGL context (which already is a FramebufferObject). The VTK scene is rendered inbetween other render items, thus the content of the external framebuffer needs to be copied to the internal framebuffer, and then back to the external framebuffer.
This blitting back to the external framebuffer is implemented, but only copies the color buffer, not the depth buffer, though in our case more external rendering might occur, which would need the depth information from the VTK scene to render correctly.
I propose to add either a flag or another enum value for FrameBlitMode to control if the depth buffer should be copied too (in vtkOpenGLRenderWindow::Frame)