add test for composite

test we can call composite before we test the result ... so we can work
with a libvips which does not have this feature

Author: jcupitt

tests/ConvenienceTest.php

@@ -44,16 +44,22 @@ public function testVipsBandsplit()
 
     public function testVipsComposite()
     {
-        /* We can't run this test on travis ... we need some way to spot
-         * composite's existence.
-         *
         $overlay = $this->image->add(20)->bandjoin(128);
         $overlay = $overlay->cast(Vips\BandFormat::UCHAR);
-        $comp = $this->image->composite($overlay, Vips\BlendMode::OVER);
 
-        $this->assertEquals($comp->getpoint(0, 0)[0], $this->pixel[0] + 10);
-         *
+        /* Added in 8.6.
          */
+        $have_composite = FALSE;
+        try {
+            $comp = $this->image->composite($overlay, Vips\BlendMode::OVER);
+            $have_composite = TRUE;
+        } catch (Exception $e) {
+        }
+
+        if ($have_composite) {
+            $comp = $this->image->composite($overlay, Vips\BlendMode::OVER);
+            $this->assertEquals($comp->getpoint(0, 0)[0], $this->pixel[0] + 10);
+        }
     }
 
     public function testVipsAddConst()