[core] coveredByChildren is false if at least one child is uncovered

[brunoabinader]

Another spin-off from #12310 - I discovered that the coveredByChildren algorithm returns true if at least one child is entirely covered by its grandchildren.

Because we are limiting the number of tiles in #12310, some edge cases were causing the lower zoom tiles to disappear - notice that symbols still appear because their rendering is unaffected by clip IDs.

On the screenshots below, notice how the uppermost tile is not rendered (only its symbols), and its absence of depth buffer color:

original	depth buffer

With the changes applied, the uppermost tile reappears, along with its depth buffer (dark grey):

fixed	depth buffer

/cc @kkaefer @asheemmamoowala

[brunoabinader]

One side effect of this change is that lower zoom tiles that are partially visible but have children covering its visible area entirely will cause coveredByChildren to return false and thus won't be removed from the clip IDs list. Though coveredByChildren logic is correct, we might need some extra logic in ClipIDGenerator::getClipIDs() to cover this corner case.

[asheemmamoowala]

nit: formatting use braces for if blocks