[Freeciv-Dev] (PR#12107) fix the isotrident mask

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=12107 >

This patch fixes the mask and the area used by "flag" graphics in 
isotrident.

Current isotrident is a little buggy this way.  In the past we've had 
problems where some pixels on the canvas weren't drawn at all.  Today if 
you look at the darkness along the ocean you can see a line of ocean 
beyond the darkness.  This is because the areas covered by the ocean and 
the darkness are not consistent.

I took the mask autogenerated by PR#12060, and applied it to all the 
base terrains.  I also applied it (actually the 32x16 one which is half 
of it) to the ocean graphics (and included a mask alongside the ocean 
graphics).  I replaced the old mask  images with the new one.  I fiddled 
with darkness a little to make it match the mask.  I also fiddled with 
irrigation and farmland.

This duplicates some of the work that Hogne (aka Freim) has been doing. 
  But this is intended to be a complete makeover that can be included in 
2.0.

-jason