Bring back layout map render / cache / rectangle functionality

[marisn]

Print layouts do not have any more option to set map rendering to cached or plain rectangle. According to ndawson, it is not needed any more "Because the map renders in the background, it no longer causes the "hangs" while rendering": https://gis.stackexchange.com/a/305631

Nevertheless constant rerendering is causing updates from remote servers (data caps, anyone?) and, on systems with less CPUs than ndawson has, it causes major slowdown of whole system as the rendering process has to run anyway instead of just rendering the final product when layouting is complete.

Tested with: QGIS versija | 3.8.1-Zanzibar | QGIS koda revīzija | 2c18e79995 on a Gentoo AMD64 with Core 2 Duo CPU and only 4GB RAM.

PS. Last month I had QGIS training in a country side school. My system (yes, Core2Duo) was fairly powerful in comparison to some other training participant systems!

[alafr]

I also noticed that the rendering of a map in the layout is about 2x slower than the rendering of the same map with the same theme in the main canvas, and it's more pixelated at high zool levels. It looks like the layouts use a different less performant rendering engine? On my PC it doesn't block or slow down the other items in the layout but it's annoying when trying to adjust the zoom level and extent of the map item itself.

[antoniolocandro]

I think the concept of "pausing" the rendering to only show the rectangles would be beneficial in some cases where you do not want to render the layout at all, so even if the feature is not needed from a performance degrade in theory, it might still benefit users that do not require the rendering initially for specific cases.