[Jprogramming] backends for gl2 emulation

[bill lam]

base.cfg (accessible from Menu Edit|Configure|base) now contains a new
global setting GL2Backend which is default to 0.

NB. GL2Backend: backend for gl2 emulation
NB.   0   system default (cairo)
NB.   1   cairo
NB.   2   gtkpixmap
NB.   3   gdiplus        (windows only)
NB.
GL2Backend=: 0

default is 0 (cairo) which is quite good but can be slow. On setting it to
2 (gtkpixmap) , then plot and other packages depending on gl2 may be 
100% faster.

gdiplus (most gl2 draw primitives implemented by Ric, thanks Ric) is available
in windows only, which can be a bit faster than pixmap. Setting it to 3 in
linux is not an error but will fall back to 0 (default cairo).

There are still some issues in gdiplus backend including bold or italic
fonts.  Please test and report if there will be other issues. A way is run
plot demo in J602 and compare the outcome with that running in 701
load '~addons/demos/plot/run.ijs'

-- 
regards,
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