* removed the global uiFrontbuf from interface.c (and python, it was used

there)
* replaced with nice local (uiBlock) storage of what goes on in drawing.
  it now only calls a glDrawBuffer() and glFinish() when it is actually
  needed

Result: interface drawing in general is speedy again, especially for gfx
cards that dont allow frontbuffer drawing, and copy stuff to the frontbuf
with a glFinish() call.

Needs to be tested on all platforms... report to me when you see problems
like menus not drawing correctly, tooltips not drawing or not disappearing,
etc.

source/blender/include/BIF_interface.h

@@ -40,8 +40,6 @@ struct ScrArea;
 typedef struct uiBut uiBut;
 typedef struct uiBlock uiBlock;
 
-extern int UIfrontbuf;
-
 void uiEmboss(float x1, float y1, float x2, float y2, int sel);
 void uiDrawMenuBox(float minx, float miny, float maxx, float maxy);
 void uiTextBoundsBlock(uiBlock *block, int addval);