Optimize paint data

To improve the performance in the data paint processing, some changes has been made in plot_item.py :

• cumulative data symbol are now created as a multi geometry instead of single geometry. It allow to call only one time renderFeature function. Futhermore geometries are created has WKB.
• cumulative data are now filtered on the displayed windows. We render only the data that we will display
• in case of huge amount of record displayed, we decimate records. This behavior will be improved in the future.