PagedLOD child selection fails when the paged database is massively scaled down.

[robertosfield]

To reproduce the issue run:

vsgdynamicload models/openstreetmap.vsgt

Eventually too many tiles will be requested and the Vulkan driver will eventually report running out of memory:

DatabasePager::updateSceneGraph() numActiveRequests = 7161 DatabasePager::updateSceneGraph() numActiveRequests = 7165 radv/amdgpu: Not enough memory for command submission. Context::waitForCompletion() 0x7f7c4c2cfcc0 fence->wait() failed with error. VkResult = -4 Context::waitForCompletion() 0x7f7c4c2cfcc0 fence->wait() failed with error. VkResult = -4 Context::waitForCompletion() 0x7f7c4c2cfcc0 fence->wait() failed with error. VkResult = -4