[GTK] Initialize text-antialias when initializing GC object

On some platforms, the default text-antialias of the system might differ
from Cairo. When a new GC object is created, the strings painted with
the initial font options no longer match the strings painted after
calling setTextAntialias(SWT.DEFAULT).

This problem primarily exists for images. For this case,
setTextAntialias(SWT.DEFAULT) should always be called to.

Author: ptziegler

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/GC.java

@@ -2659,6 +2659,9 @@ void init(Drawable drawable, GCData data, long gdkGC) {
 	Cairo.cairo_set_fill_rule(cairo, Cairo.CAIRO_FILL_RULE_EVEN_ODD);
 	data.state &= ~(BACKGROUND | FOREGROUND | FONT | LINE_WIDTH | LINE_CAP | LINE_JOIN | LINE_STYLE | DRAW_OFFSET);
 	setClipping(data.clipRgn);
+	if (image != null) {
+		setTextAntialias(SWT.DEFAULT);
+	}
 	initCairo();
 	if ((data.style & SWT.MIRRORED) != 0) {
 		// Don't overwrite the Cairo transformation matrix in GTK 3.14 and above; it contains a translation relative to the parent widget.

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_graphics_Image.java

@@ -1257,4 +1257,46 @@ public void test_updateWidthHeightAfterDPIChange() {
 		DPIUtil.setDeviceZoom(deviceZoom);
 	}
 }
+
+@Test
+public void test_paintWithTextAntialias() {
+	// Must be executed on e.g. a Gnome system
+	// where anti-alias is set to use Grayscale
+	// whereas Cairo is using RGB by default
+	int[] modes = {SWT.ON, SWT.OFF, SWT.DEFAULT};
+	int width = (modes.length + 1) * 100;
+	int height = 100;
+
+	Image image1 = new Image(display, width, height);
+	GC g1 = new GC(image1);
+	g1.setAdvanced(true);
+
+	g1.drawString("OWVO", 35, 20);
+	for (int i = 0 ; i < modes.length; ++i) {
+		g1.setTextAntialias(modes[i]);
+		g1.drawString("OWVO", 135 + i * 100, 20);
+	}
+
+	Image image2 = new Image(display, width, height);
+	GC g2 = new GC(image2);
+	g2.setAdvanced(true);
+
+	for (int i = modes.length - 1 ; i >= 0; --i) {
+		g2.setTextAntialias(modes[i]);
+		g2.drawString("OWVO", 135 + i * 100, 20);
+	}
+	g2.setTextAntialias(SWT.DEFAULT);
+	g2.drawString("OWVO", 35, 20);
+
+	ImageData data1 = image1.getImageData();
+	ImageData data2 = image2.getImageData();
+
+	g1.dispose();
+	g2.dispose();
+	image1.dispose();
+	image2.dispose();
+
+	ImageTestUtil.assertImagesEqual(data1, data2);
+}
+
 }