Compare actual RGB values when comparing image datas with palette data

For image datas without palette data (direct palettes) the code before
worked ok because the getPixel's returned value was the direct color.
But for palette images, the getPixel returns the index.

When an index is used, you need to call getRGB to get the real data
value.

Part of #2126

Author: jonahgraham

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/graphics/ImageDataTestHelper.java

@@ -223,7 +223,9 @@ public static Comparator<ImageData> imageDataComparator() {
 				.thenComparing((ImageData firstData, ImageData secondData) -> {
 					for (int x = 0; x < firstData.width; x++) {
 						for (int y = 0; y < firstData.height; y++) {
-							if (firstData.getPixel(x, y) != secondData.getPixel(x, y)) {
+							RGB first = firstData.palette.getRGB(firstData.getPixel(x, y));
+							RGB second = secondData.palette.getRGB(secondData.getPixel(x, y));
+							if (!first.equals(second)) {
 								return -1;
 							}
 							if (firstData.getAlpha(x, y) != secondData.getAlpha(x, y)) {