r_wateralpha integration for mappers

[F0mx]

Description

Add a worldspawn entity property that allows mappers to dictate r_wateralpha on map start, which would allow for far greater control of water details in maps. Currently, water is defaulted to completely opaque, however r_wateralpha is already supported and can be used to adjust water transparency via command line.

Being able to adjust this value allows mappers to match water in maps to fit the lighting and atmosphere they are looking to create, where I feel giving a stock option of either transparent or opaque (say 1 and 0.5) would reduce these options considerably.
Below are some examples of this:

r_wateralpha 1

r_wateralpha 0.5

r_wateralpha 0.1

r_wateralpha 0.025

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