Performance enhancing for footprint rendering (polygons)

[bmatthieu3]

Above 3000/4000 polygon footprints there is performance issues. Several optimization could be done:

• Render the footprints with GPU (line GPU rasterizer using instancing see these article/topics https://wwwtyro.net/2019/11/18/instanced-lines.html, https://mattdesl.svbtle.com/drawing-lines-is-hard, https://dl.acm.org/doi/10.1145/1179849.1180035). This will benefit all things rendered from lines e.g. moc HPX cell edges or the grid that uses lines.
• Do the vertices projection on the GPU side (beware of the precision issues that might come because operations are done on float in GPU and not double, to be tested...)
• Index the footprints in HEALPix cells so that only footprints inside HPX cells in the view will be rendered