[core] Ensure correct GL context in Map::onLowMemory()

[jfirebaugh]

Fixes #5731

[mention-bot]

@jfirebaugh, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kkaefer, @brunoabinader and @1ec5 to be potential reviewers.

[jfirebaugh]

This probably fixes #1509 as well.

[mb12]

It most likely doesn't fix #1509. It's a single map with memory under 25 MB the last time I tried. The geometries are literally quads that resselate into two triangles per quad. It doesn't use any symbols etc. The vector tiles are trivial quads literally.

[brunoabinader]

Great finding, @jfirebaugh - though for future reference, it might be interesting for the Painter object to receive a reference for the whole Backend on its ctor instead of just Context, because Context can't currently activate itself. We could then have a RAII-based guard on every painter call that handles OpenGL calls, like cleanup() and reset() for instance.

[kkaefer]

FWIW, I tried to trigger this particular code path when I debugged the original issue, but couldn't get it to crash even in severely RAM constrained environments, however, I only used one map so there probably was no GL context conflict.

While we're at it: we should also move the call to style->onLowMemory() before the painter cleanup; it may release resources that are not cleaned up. We seem to be triggering a rerender, but I'm not sure this is strictly necessary. An alternative could be to not call painter->cleanup(), and exclusively rely on the cleanup call that happens as part of the repaint anyway. However, this could potentially worsen the memory situation since a redraw might potentially allocate even more memory.

[jfirebaugh]

@brunoabinader @kkaefer -- good ideas. Can you work on followup PRs for those?