[Performance] gdImageStringFT called multiple time in msDrawTextGD.

[jlacroix]

In bug 1224, I found out that in msDrawTextGD, gdImageStringFT is called 7 times
in a row to make the outline of the text. In 4.0 there were only three call to
this function. I suppose it's for a better outlining of labels.

This cause a problem, a performance lost. It depends on the mapfiles, but in the
GMap demo (around 14 labels), it results in a 15% performance lost.

With 4.0, for 50 call to gmap75.map:
total: 6.18 sec.
in gdImageStringFT: 1.66 sec.

With 4.0:
total: 7.56
in gdImageStringFT: 2.51 sec.

Is there a way to reduce the cost of outlining?
To reduce the number of call?