Better Double Slabs

Changes Slabs and Double Slabs to have clear seperation to them, similar to the Smooth Stone Slab.

Affects the following slabs:

• Polished Granite
• Polished Diorite
• Polished Andesite
• Polished Deepslate
• Polished Tuff
• Sandstone
• Cut Sandstone
• Red Sandstone
• Cut Red Sandstone
• Prismarine Bricks
• Polished Blackstone
• Quartz