Drawing widgets is time-consuming

[Rafał Cieślak]

The drawing model we currently use is extremely inefficient.

The display keeps redrawing relentlessly. Each time we process all visible widgets and redraw them. With a larger number of widgets, the interface will start lagging.

I've run valgrind's profiler, and it turns out that about 50% of code execution time is spend rendering fonts.

Why don't we render them once, and use a pre-rendered bitmap next times?
Of course, the bitmap would need to be re-rendered when the text is changed or the drawing area resizes. But that happens relatively rarely, and therefore such approach would improve our efficiency a lot.

This concept may be generalised, let all drawables have a needs_redraw flag that causes the pre-rendered "buffer" bitmap to be redrawn, and in all other cases draw() would simply use that buffer to draw onto the display.