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Fix display glitch in Jigsaw Solo's pencil-mode cursor.

In Jigsaw Solo, block boundaries aren't convex, so it's possible for
one of them to have an inward corner. If that corner is in the top
left of a selectable cell, and you right-click that cell to display
the pencil-mode 'cursor' in the form of a triangle in the top left,
then the cursor was accidentally drawn on top of the block boundary,
where it ought to be underneath it.

For example, in game id 5j:d1d4_4c3_1d2d,bb_baaa_dca_baaba, right-
clicking in the bottom right square of the grid demonstrates the
problem.

Jonas Kölker fixed this for Keen in 2015, in commit 6482ed0e3c886af.
This is the identical fix, in Solo's very similar-looking drawing
routine. I feel embarrassed to have taken eight years to get round to
it!

--- a/solo.c

+++ b/solo.c

@@ -4986,6 +4986,18 @@

 	       (ds->xtype && (ondiag0(y*cr+x) || ondiag1(y*cr+x))) ? COL_XDIAGONALS :

 	       COL_BACKGROUND));

+    /* pencil-mode highlight */

+    if ((hl & 15) == 2) {

+        int coords[6];

+        coords[0] = cx;

+        coords[1] = cy;

+        coords[2] = cx+cw/2;

+        coords[3] = cy;

+        coords[4] = cx;

+        coords[5] = cy+ch/2;

+        draw_polygon(dr, coords, 3, COL_HIGHLIGHT, COL_HIGHLIGHT);

+    }

+

     /*

      * Draw the corners of thick lines in corner-adjacent squares,

      * which jut into this square by one pixel.

@@ -4998,18 +5010,6 @@

 	draw_rect(dr, tx-GRIDEXTRA, ty+TILE_SIZE-1-2*GRIDEXTRA, GRIDEXTRA, GRIDEXTRA, COL_GRID);

     if (x+1 < cr && y+1 < cr && state->blocks->whichblock[y*cr+x] != state->blocks->whichblock[(y+1)*cr+x+1])

 	draw_rect(dr, tx+TILE_SIZE-1-2*GRIDEXTRA, ty+TILE_SIZE-1-2*GRIDEXTRA, GRIDEXTRA, GRIDEXTRA, COL_GRID);

-

-    /* pencil-mode highlight */

-    if ((hl & 15) == 2) {

-        int coords[6];

-        coords[0] = cx;

-        coords[1] = cy;

-        coords[2] = cx+cw/2;

-        coords[3] = cy;

-        coords[4] = cx;

-        coords[5] = cy+ch/2;

-        draw_polygon(dr, coords, 3, COL_HIGHLIGHT, COL_HIGHLIGHT);

-    }

     if (state->kblocks) {

 	int t = GRIDEXTRA * 3;