[MC-3927] Iron golems spawning in slabs, glass, farmland and grass path

Risk of confusion

This report describes only iron golems spawning inside non-opaque non-full cubes.

See ~~MC-130270~~ for them spawning even inside opaque full cubes, which started during the 1.13 snapshots.

This issue reappeared in 1.14, see ~~MC-148955~~ for more details.

The bug

Iron golems can spawn in blocks which are considered as not normal cube (normal cubes are: state.getMaterial().isOpaque() && state.isFullCube() && !state.canProvidePower()).

This makes them spawn in slabs, glass, farmland, grass path, ...

How to reproduce

Create village with villagers
Fill all air blocks at the village center with for example glass
Wait for an iron golem to spawn

Code analysis

Based on 1.12.2 decompiled using MCP 9.40

The method net.minecraft.village.Village.isAreaClearAround(BlockPos, BlockPos) only considers a spawn location as invalid if the blocks at this position are net.minecraft.block.state.IBlockProperties.isNormalCube(), it should probably instead use logic similar to the one used in net.minecraft.world.WorldEntitySpawner.findChunksForSpawning(WorldServer, boolean, boolean, boolean).