The pngcairo terminal was broken for wxdraw2d. Made it work again.

Closes #323

The variable wxplot_pngcairo that allows to tell wxplot and wxdraw to use the newer pngcairo terminal instead of the old basic png one was introduced in 2011: https://www.ma.utexas.edu/pipermail/maxima/2011/023696.html

Unfortunately it hase ceased to work for wxdraw2d, since.

This patch re-enables this feature.