Prevent values like -0 in xdot output

Two graphs in the shapes regression test reference files (invhouse and
invtrapezium) contained a -0 coordinate. The Linux build produced the same
results, but the Windows build didn't, causing the regression tests to
fail.
To prevent values like -0 in the xdot output, a comparison is added just
before formatting the number.

plugin/core/gvrender_core_dot.c

@@ -140,6 +140,11 @@ static void xdot_trim_zeros (char* buf, int addSpace)
  */
 static void xdot_fmt_num (char* buf, double v)
 {
+    // Prevents values like -0
+    if (v > -0.00000001 && v < 0.00000001)
+    {
+        v = 0;
+    }
     sprintf(buf, "%.02f", v);
     xdot_trim_zeros (buf, 1);
 }

tests/regression_tests/shapes/reference/invhouse.xdot

@@ -4,7 +4,7 @@ graph G {
 		xdotversion=1.7
 	];
 	node [label="\N"];
-	a	 [_draw_="c 7 -#000000 p 5 0 12.44 27 -0 54 12.44 53.97 32.56 0.03 32.56 ",
+	a	 [_draw_="c 7 -#000000 p 5 0 12.44 27 0 54 12.44 53.97 32.56 0.03 32.56 ",
 		height=0.5,
 		label="",
 		pos="27,18",

tests/regression_tests/shapes/reference/invtrapezium.xdot

@@ -4,7 +4,7 @@ graph G {
 		xdotversion=1.7
 	];
 	node [label="\N"];
-	a	 [_draw_="c 7 -#000000 p 4 11.23 0 42.77 0 54 36 -0 36 ",
+	a	 [_draw_="c 7 -#000000 p 4 11.23 0 42.77 0 54 36 0 36 ",
 		height=0.5,
 		label="",
 		pos="27,18",