Fix for bug #8324: J-key render buffer switching had various issues.

One problem was that the previous render buffer was stored in the
render window or image window itself, which means that when closing
it, or switching from one to the other, things didn't work as expected.
Also in the image editor, color sampling the previous render buffer or
saving it didn't work correct.

source/blender/include/BIF_renderwin.h

@@ -33,6 +33,7 @@
 struct Render;
 struct ScrArea;
 struct RenderStats;
+struct ImBuf;
 
 void calc_renderwin_rectangle(int rectx, int recty, int posmask, int renderpos_r[2], int rendersize_r[2]);
 
@@ -40,6 +41,15 @@ void BIF_close_render_display(void);
 
 void BIF_do_render(int anim);
 
+/* render text */
+void BIF_make_render_text(struct RenderStats *rs);
+char *BIF_render_text(void);
+
+/* render spare buffer */
+int BIF_show_render_spare(void);
+struct ImBuf *BIF_render_spare_imbuf(void);
+void BIF_free_render_spare(void);
+
 /**
  * @param v3d The View3D space to render.
  */