(svn r9371) -Feature: Add support for variable snow lines in the arctic climate, supplied

by newgrf files. When this is enabled forests cannot be built below the highest
snow line, and farms can't be built above it. Houses still use the
_opt.snow_line so they are all consistent, so to make them respect the snowline
you may want to use some newhouses features as well.

src/landscape.h

@@ -0,0 +1,19 @@
+/* $Id$ */
+
+/** @file landscape.h */
+
+enum {
+	SNOW_LINE_MONTHS = 12,
+	SNOW_LINE_DAYS   = 32,
+};
+
+struct SnowLine {
+	byte table[SNOW_LINE_MONTHS][SNOW_LINE_DAYS];
+	byte highest_value;
+};
+
+bool IsSnowLineSet(void);
+void SetSnowLine(byte table[SNOW_LINE_MONTHS][SNOW_LINE_DAYS]);
+byte GetSnowLine(void);
+byte HighestSnowLine(void);
+void ClearSnowLine(void);