fix MC-197260 causing armorstands to render dark

[MrKinau]

Fixes #507

Root cause of this issue is that skylight is taken from the block with the highest block light. If all block lights are 0, the sky light might be taken from a block where the sky light is 0 as well (e.g. the block below the armorstand). Changed it to always merge all checked blocks: Using the highest sky light and highest block light.