Render parent raster tiles when tiles can't be loaded

[kkaefer]

In #8164, we implemented raster tile overzooming. However, this fails in non-continuous rendering modes ("Still image rendering").

I believe this is a combination of two bugs, one of them in the node bindings, the other in updateRenderables:

• Node FileSource doesn't support optional, so the code path for loading parent tiles is never triggered, and all tiles we attempt to load are not in complete state,
• Still image rendering relies on tiles being marked as complete, but that never happens when a tile can't be loaded. This isn't an issue for continuous rendering because we just render what we have, but it's a termination condition for Still image rendering.

JS test is in mapbox/mapbox-gl-js#4612

/cc @tmpsantos @bsudekum

[bsudekum]

@kkaefer are there any platform specific implications worth noting with this change?

[kkaefer]

are there any platform specific implications worth noting with this change?

It changes the behavior in two ways:

• We are now triggering "required" requests after a parent tile failed to load. Previously, we only triggered optional requests, so file sources that don't support these never loaded parent tiles.
• We are marking tiles that we attempted to load, but that failed to load or parse as "complete/loaded" (but not "renderable"). This means that they now trigger loading parent tiles, and allow the overall map to transition into a "loaded" state, which is the termination condition for Still image rendering.

I believe that this change doesn't alter any visible behavior for continuous rendering mode.