* 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/interface.h

@@ -43,7 +43,6 @@
 #define UI_HELV			0
 #define UI_HELVB		1
 
-
 /* Button types */
 #define CHA	32
 #define SHO	64