Feature: setting to indicate snow coverage for arctic climate (replaces snow line height)

Setting the snow coverage (in % of the map) makes a lot more sense
to the human, while still allowing the niche player to set (by
finding the correct %) a snow line height they like. This makes for
easier defaults, as it decoupled terrain height from amount of snow.

Maps can never be 100% snow, as we do not have sprites for coastal
tiles.

Internally, this calculates the best snow line height to approach
this coverage as close as possible.

src/table/settings.ini

@@ -1408,7 +1408,7 @@ cat      = SC_BASIC
 base     = GameSettings
 var      = game_creation.snow_line_height
 type     = SLE_UINT8
-guiflags = SGF_NO_NETWORK
+guiflags = SGF_SCENEDIT_ONLY
 def      = DEF_SNOWLINE_HEIGHT
 min      = MIN_SNOWLINE_HEIGHT
 max      = MAX_SNOWLINE_HEIGHT
@@ -1418,6 +1418,21 @@ strhelp  = STR_CONFIG_SETTING_SNOWLINE_HEIGHT_HELPTEXT
 strval   = STR_JUST_COMMA
 cat      = SC_BASIC
 
+[SDT_VAR]
+base     = GameSettings
+var      = game_creation.snow_coverage
+type     = SLE_UINT8
+flags    = SLF_NOT_IN_SAVE | SLF_NO_NETWORK_SYNC
+guiflags = SGF_NEWGAME_ONLY
+def      = DEF_SNOW_COVERAGE
+min      = 0
+max      = 100
+interval = 10
+str      = STR_CONFIG_SETTING_SNOW_COVERAGE
+strhelp  = STR_CONFIG_SETTING_SNOW_COVERAGE_HELPTEXT
+strval   = STR_CONFIG_SETTING_SNOW_COVERAGE_VALUE
+cat      = SC_BASIC
+
 [SDT_NULL]
 length   = 4
 to       = SLV_144