Add uniform for freezing effect

[kolgushev]

Is your feature request related to a problem?
I would like to add a vignette distortion effect to my shader when players freeze in powdered snow.

Describe the solution you'd like
A uniform float currentPlayerFreezing ranging 0-1, where 0 is no freezing and 1 is taking damage from freezing (and custom health hearts texture)

Describe alternatives you've considered
The effect can be emulated by creating a custom uniform representing a smoothed isEyeInWater == 3, but this 1) doesn't scale linearly over time, and 2) doesn't work when only the player's feet are in the powdered snow.