Add getframe function for opengl based toolkits (bug #48195) * graphics.in.h (graphics_toolkit::get_pixels, base_graphics_toolkit::get_pixels): new virtual methods * graphics.cc (F__get_frame__): new function. * Backend.h/cc (Backend::get_pixels): implement new method * ObjectProxy.h/cc (ObjectProxy::get_pixels): new method, invoke directly Figure object slotGetPixels in Qt::BlockingQueuedConnection mode * Figure.h/cc (Figure::slotGetPixels): new public slot * Canvas.h (Canvas::getPixels, Canvas::do_getPixels): new methods * GLCanvas.h/cc (GLCanvas::do_getPixels): reimplemented method, use opengl_renderer to draw object and get_pixels * __init_fltk__.cc (fltk_graphics_toolkit::get_pixels, figure_manager::get_pixels, figure_manager::do_get_pixels, plot_window::get_pixels, OpenGL_fltk::get_pixels): new methods * gl-renderer.h/cc (opengl_renderer::get_pixels): new method, read pixels in the current opengl scene * getframe.m: new function file * image.txi: add getframe doc * NEWS: announce new function

--- a/scripts/image/frame2im.m	Sat May 27 09:59:32 2017 -0700
+++ b/scripts/image/frame2im.m	Tue May 23 15:01:27 2017 +0200
@@ -28,7 +28,7 @@
 ## for indexed and RGB movies respectively, with each frame concatenated
 ## along the 4th dimension.
 ##
-## @seealso{im2frame}
+## @seealso{im2frame, getframe}
 ## @end deftypefn
 
 ## Author: Carnë Draug <carandraug@octave.org>