Fix T77564: VSE (and compositor background) lost stereoscopy preview

Issue introduced on fe045b2b77.

Since the stereoscopy compositing (anaglyph, ...) is only done for
viewports the VSE preview and compositor need to use viewports.

Reviewed by: dfelinto

Differential Revision: https://developer.blender.org/D8472

source/blender/editors/space_node/node_draw.c

@@ -52,6 +52,7 @@
 #include "GPU_immediate_util.h"
 #include "GPU_matrix.h"
 #include "GPU_state.h"
+#include "GPU_viewport.h"
 
 #include "WM_api.h"
 #include "WM_types.h"
@@ -1755,6 +1756,12 @@ void drawnodespace(const bContext *C, ARegion *region)
   SpaceNode *snode = CTX_wm_space_node(C);
   View2D *v2d = &region->v2d;
 
+  /* Setup offscreen buffers. */
+  GPUViewport *viewport = WM_draw_region_get_viewport(region);
+
+  GPUFrameBuffer *framebuffer_overlay = GPU_viewport_framebuffer_overlay_get(viewport);
+  GPU_framebuffer_bind_no_srgb(framebuffer_overlay);
+
   UI_ThemeClearColor(TH_BACK);
   GPU_clear(GPU_COLOR_BIT);