feat: render relevant parts of the screen only

[sassanh]

feat: keep a copy of the raw data transferred to the display in config._display.raw_data

This is useful for snapshot tests to snapshot exactly what is rendered on the screen.

This pull request also finalizes partial renders.

[mehrdadfeller]

looks good! you can go ahead and merge. I just have sone question: does this "render relevant parts of the screen only" part happen in display.py? does the changes inside the rectangle only get transfered to the display?

[sassanh]

@mehrdadfeller No, it only makes each widget render only its own rectangle to the display. With this pull request headless-kivy-pi provides everything needed for partial rendering, but ubo-gui still needs to use HeadlessWidgets for all its components instead of regular kivy Widgets.