title: find all inner grid points of a polygon made up from neighbouring grid points
body: <p>I have list of Points (int x, int y).
  together they form areas, I check if this area is closed and then I need to get inner area formed by all positions that are inside this area.</p>
  
  <p>example area:</p>
  
  <p><img src="https://i.stack.imgur.com/FemJH.png" alt="enter image description here"></p>
  
  <p>Only idea I had is to convert this area to vector and check every point if it is inside polygon or not, counting intersections of polygon a axis's of point.</p>
  
  <p>But I don't think it would be the most efficient way to do it.</p>
  
  <p>other idea was to first get all points that are outside, I start from corners (if corner is not part of list of points, then is 100% empty), add all neighbor points that are empty and repeat.
  then all points that aren't outside and aren't in highlighted list are inside.</p>
  
  <p>but again, it feels somehow cumbersome...</p>