Map Tools - Fix Straight Line Drawing being broken by Higher Draw Priority

[mrschick]

When merged this pull request will:

• Fixes an issue where lines drawn along a map tool would not be straightened if the mission had an editor-placed marker with a draw priority higher than 0.
  As mentioned here, since 2.18 allMapMarkers is ordered according to priority first, instead of creation. That causes those markers with higher priority to always be at the end of the array.
  The current code however still assumes that allMapMarkers select -1 is always the line that was just drawn, causing it to compare the cached GVAR(drawPosStart) with the wrong marker and always aborting line straightening due to the distance check.