-- expects globals grid[], gw, gh
-- uses global r
-- stack layout: {{x, y}, ...}

function fill(stack)
  while #stack > 0 do
    local x, y = unpack(stack[#stack])
    stack[#stack] = nil
    
    if not (x < 0 or y < 0 or x >= gw or y >= gh) then
      local idx = y*gw+x+1
      
      if grid[idx] then
        grid[idx] = nil
        local me = newRectangle(x, y, 1, 1)
        if r == nil then
          r = me
        else
          r = mergeRectangles(r, me)
        end
        
        stack[#stack+1] = {x-1, y}
        stack[#stack+1] = {x+1, y}
        stack[#stack+1] = {x, y-1}
        stack[#stack+1] = {x, y+1}
      end
    end
  end
end