Fix #107726: Viewport animation render ignores start frame

This issue specifically happend if sound playback is synchronized by
Audaspace. In this case render engine can't override current frame.
`BKE_sound_sync_scene()` should return NaN as when rendering is in
progress, however viewport render operator doesn't set `G.is_rendering`.

Pull Request: https://projects.blender.org/blender/blender/pulls/111946

source/blender/editors/render/render_opengl.cc

@@ -935,6 +935,7 @@ static void screen_opengl_render_end(bContext *C, OGLRender *oglrender)
   CTX_wm_region_set(C, oglrender->prevar);
 
   MEM_delete(oglrender);
+  G.is_rendering = false;
 }
 
 static void screen_opengl_render_cancel(bContext *C, wmOperator *op)
@@ -992,6 +993,7 @@ static bool screen_opengl_render_anim_init(bContext *C, wmOperator *op)
     }
   }
 
+  G.is_rendering = true;
   oglrender->cfrao = scene->r.cfra;
   oglrender->nfra = PSFRA;
   scene->r.cfra = PSFRA;