#18 BarGlass fixes

[João Fonseca]

Hi,

I'm using BarGlass charts, and found some slight rendering annoyances:

-Zero values are drawn anyway, with odd-looking inverted rounded corners.

-When the bars are very thin (for example, if there are many bars and low horizontal space), a wider line is drawn at the top of each bar.

I patched BarGlassStyle.as, solving these issues. Use in
open-flash-chart-1.9.7

[João Fonseca]