Add tilemap.get_tiles() function

[AGulev]

Added a new function tilemap.get_tiles() which returns a table of rows with all the tile indexes for the layer.

local left, bottom, columns_count, rows_count = tilemap.get_bounds("#tilemap")
local tiles = tilemap.get_tiles("#tilemap", "layer")
local tile, count = 0, 0
for row_index = bottom, bottom + rows_count - 1 do
	for column_index = left, left + columns_count - 1 do
		tile = tiles[row_index][column_index]
		count = count + 1
	end
end

[britzl]

Looks good! But we need to document the order in the returned tiles table.