Re-organize grid management code for less dependance between files

Remove gridio.lua and move its function to appropriate files

pregenerate.lua

@@ -1,6 +1,8 @@
 -- Generate the grid using terrainlib_lua
 -- Only called on first mapgen, if there is no grid yet
 
+-- Constants
+
 local EvolutionModel = dofile(mapgen_rivers.modpath .. '/terrainlib_lua/erosion.lua')
 local twist = dofile(mapgen_rivers.modpath .. '/terrainlib_lua/twist.lua')
 
@@ -52,12 +54,10 @@ local function margin(dem, width, elev)
 	end
 end
 
--- Generate the grid
+-- Generate grid
 
 minetest.log("action", '[mapgen_rivers] Generating grid, this may take a while...')
 
-local grid = {size={x=X, y=Y}}
-
 if X*Y > 4e6 then
 	minetest.log("warning", "[mapgen_rivers] You are going to generate a very large grid (>4M nodes). If you experience problems, you should increase blocksize or reduce map size.")
 end
@@ -103,19 +103,48 @@ model:flow()
 
 local mfloor = math.floor
 local mmin, mmax = math.min, math.max
+local unpk, schar = unpack, string.char
 local offset_x, offset_y = twist(model.dirs, model.rivers, 5)
 for i=1, X*Y do
 	offset_x[i] = mmin(mmax(offset_x[i]*256, -128), 127)
 	offset_y[i] = mmin(mmax(offset_y[i]*256, -128), 127)
 end
 
-grid.dem = model.dem
-grid.lakes = model.lakes
-grid.dirs = model.dirs
-grid.rivers = model.rivers
-grid.offset_x = offset_x
-grid.offset_y = offset_y
+-- Write data
 
-collectgarbage()
+local datapath = mapgen_rivers.world_data_path
+minetest.mkdir(datapath)
 
-return grid
+local sfile = io.open(datapath .. 'size', "w")
+sfile:write(X..'\n'..Y)
+sfile:close()
+
+local function write_file(filename, data, bytes)
+	local file = io.open(datapath .. filename, 'wb')
+
+	local bytelist = {}
+	for j=1, bytes do
+		bytelist[j] = 0
+	end
+
+	for i=1, #data do
+		local n = mfloor(data[i])
+		data[i] = n
+		for j=bytes, 2, -1 do
+			bytelist[j] = n % 256
+			n = mfloor(n / 256)
+		end
+		bytelist[1] = n % 256
+
+		file:write(schar(unpk(bytelist)))
+	end
+
+	file:close()
+end
+
+write_file('dem', model.dem, 2)
+write_file('lakes', model.lakes, 2)
+write_file('dirs', model.dirs, 1)
+write_file('rivers', model.rivers, 4)
+write_file('offset_x', offset_x, 1)
+write_file('offset_y', offset_y, 1)