Better Snow Coverage

• fix snow being rendered on top of fence - #19

• fix rare crash with sodium - #14

• fix rare crash with sodium - #16 (August Saint Freytag)
• add optional texturepack to improve z-fighting

• support 1.21.5

• add optional texturepack to improve z-fighting - #12 (bearofbusiness)

• fix iris support
• add optional texturepack to improve z-fighting - #9
• expand settings - #9

backport to 1.20.2 - bearofbusiness

backport to 1.20.1

support sodium 0.6 improve snow layer placement

improve decision-making for where to show snow - see github