Above water terrain generation needs fixing

Multiple problems with terrain including-

• Floating blocks/islands
• High walls and corners on chunk borders
• Entire chunks missing snow or ice
• Floating grass
• Odd mineshaft generation in mesa biomes
• Snow and desert biome generation next to each other

Everything seem too course and needs to be smoothed out a little.

All screenshots taken by myself was done in a single freshly generated world using Minecraft 1.13-pre1 with seed: 1453522920