Fix PHP bug #64641: imagefilledpolygon doesn't draw horizontal line

As reported in <https://bugs.php.net/64641> 1-dimensional horizontal
filled polygons are not drawn at all. That is caused by the scanline
algorithm used for drawing filled polygons which skips the drawing of
horizontal edges, because that is normally not necessary. If, however,
the polygon consists of horizontal edges only, that obviously doesn't
work, so we add a special case handling.

src/gd.c

@@ -3142,6 +3142,19 @@ BGD_DECLARE(void) gdImageFilledPolygon (gdImagePtr im, gdPointPtr p, int n, int
 			maxy = p[i].y;
 		}
 	}
+	/* necessary special case: horizontal line */
+	if (n > 1 && miny == maxy) {
+		x1 = x2 = p[0].x;
+		for (i = 1; (i < n); i++) {
+			if (p[i].x < x1) {
+				x1 = p[i].x;
+			} else if (p[i].x > x2) {
+				x2 = p[i].x;
+			}
+		}
+		gdImageLine(im, x1, miny, x2, miny, c);
+		return;
+	}
 	pmaxy = maxy;
 	/* 2.0.16: Optimization by Ilia Chipitsine -- don't waste time offscreen */
 	/* 2.0.26: clipping rectangle is even better */

tests/gdimagefilledpolygon/.gitignore

@@ -3,3 +3,4 @@
 /gdimagefilledpolygon1
 /gdimagefilledpolygon2
 /gdimagefilledpolygon3
+/php_bug_64641

tests/gdimagefilledpolygon/CMakeLists.txt

@@ -4,6 +4,7 @@ LIST(APPEND TESTS_FILES
 	gdimagefilledpolygon2
 	gdimagefilledpolygon3
 	bug00100
+	php_bug_64641
 )
 
 ADD_GD_TESTS()

tests/gdimagefilledpolygon/Makemodule.am

@@ -4,7 +4,8 @@ libgd_test_programs += \
 	gdimagefilledpolygon/gdimagefilledpolygon0 \
 	gdimagefilledpolygon/gdimagefilledpolygon1 \
 	gdimagefilledpolygon/gdimagefilledpolygon2 \
-	gdimagefilledpolygon/gdimagefilledpolygon3
+	gdimagefilledpolygon/gdimagefilledpolygon3 \
+	gdimagefilledpolygon/php_bug_64641
 endif
 
 EXTRA_DIST += \
@@ -13,4 +14,5 @@ EXTRA_DIST += \
 	gdimagefilledpolygon/gdimagefilledpolygon0.png \
 	gdimagefilledpolygon/gdimagefilledpolygon1.png \
 	gdimagefilledpolygon/gdimagefilledpolygon2.png \
-	gdimagefilledpolygon/gdimagefilledpolygon3.png
+	gdimagefilledpolygon/gdimagefilledpolygon3.png \
+	gdimagefilledpolygon/php_bug_64641.png

tests/gdimagefilledpolygon/php_bug_64641.c

@@ -0,0 +1,47 @@
+/*
+	Test drawing of 1-dimensional filled polygons
+
+	We're drawing a vertical and a horizontal 1-dimensional filled polygon,
+	which is supposed to result in a vertical and a horizontal line.
+
+	See also <https://bugs.php.net/64641>.
+*/
+
+#include "gd.h"
+#include "gdtest.h"
+
+int main()
+{
+	gdImagePtr im;
+	gdPointPtr points;
+
+	im = gdImageCreateTrueColor(640, 480);
+
+	points = (gdPointPtr)calloc(3, sizeof(gdPoint));
+	gdTestAssert(points != NULL);
+
+	/* vertical line */
+	points[0].x = 100;
+	points[0].y = 100;
+	points[1].x = 100;
+	points[1].y = 200;
+	points[2].x = 100;
+	points[2].y = 300;
+	gdImageFilledPolygon(im, points, 3, 0xFFFF00);
+
+	/* horizontal line */
+	points[0].x = 300;
+	points[0].y = 200;
+	points[1].x = 400;
+	points[1].y = 200;
+	points[2].x = 500;
+	points[2].y = 200;
+	gdImageFilledPolygon(im, points, 3, 0xFFFF00);
+
+	gdAssertImageEqualsToFile("gdimagefilledpolygon/php_bug_64641.png", im);
+
+	free(points);
+	gdImageDestroy(im);
+
+	return gdNumFailures();
+}