8354191: GTK LaF should use pre-multiplied alpha same as cairo

Reviewed-by: avu, prr

Author: Maxim Kartashev

src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/GTKEngine.java

@@ -532,14 +532,7 @@ public void paintBackground(Graphics g, SynthContext context,
         native_paint_background(widget, state, x - x0, y - y0, w, h);
     }
 
-    private static final ColorModel[] COLOR_MODELS = {
-        // Transparency.OPAQUE
-        new DirectColorModel(24, 0x00ff0000, 0x0000ff00, 0x000000ff, 0x00000000),
-        // Transparency.BITMASK
-        new DirectColorModel(25, 0x00ff0000, 0x0000ff00, 0x000000ff, 0x01000000),
-        // Transparency.TRANSLUCENT
-        ColorModel.getRGBdefault(),
-    };
+    private static final ColorModel[] COLOR_MODELS = new ColorModel[3];
 
     private static final int[][] BAND_OFFSETS = {
         { 0x00ff0000, 0x0000ff00, 0x000000ff },             // OPAQUE
@@ -609,15 +602,27 @@ public BufferedImage finishPainting(boolean useCache) {
         WritableRaster raster = Raster.createPackedRaster(
                 dataBuffer, w0, h0, w0, bands, null);
 
-        ColorModel cm = COLOR_MODELS[transparency - 1];
-        BufferedImage img = new BufferedImage(cm, raster, false, null);
+        ColorModel cm = colorModelFor(transparency);
+        BufferedImage img = new BufferedImage(cm, raster, true, null);
         if (useCache) {
             cache.setImage(getClass(), null, w0, h0, cacheArgs, img);
         }
         graphics.drawImage(img, x0, y0, null);
         return img;
     }
 
+    private ColorModel colorModelFor(int transparency) {
+        synchronized (COLOR_MODELS) {
+            int index = transparency - 1;
+            if (COLOR_MODELS[index] == null) {
+                COLOR_MODELS[index] = GraphicsEnvironment.getLocalGraphicsEnvironment()
+                        .getDefaultScreenDevice().getDefaultConfiguration()
+                        .getColorModel(transparency);
+            }
+            return COLOR_MODELS[index];
+        }
+    }
+
     /**
      * Notify native layer of theme change, and flush cache
      */