Correct wrongly scaled images in colors/fonts preference page

The colors/fonts preference page uses images for the items shown in a
tree viewer that preview the selected color. The current implementation
has two limitations:
- It does not properly scale with the current monitor zoom
- It uses a wrong image size based on the tree item height instead of
the tree item image size, such that the rendered image is scaled and may
loose information

This change removes all custom extraction of the image size as the image
simply needs to have the same size as all other images default to
(16x16) to fit. It also wraps the implementation info an ImageGcDrawer
to be monitor-scale-aware.

Author: arunjose696

bundles/org.eclipse.ui.workbench/eclipseui/org/eclipse/ui/internal/themes/ColorsAndFontsPreferencePage.java

@@ -63,6 +63,7 @@
 import org.eclipse.swt.graphics.FontMetrics;
 import org.eclipse.swt.graphics.GC;
 import org.eclipse.swt.graphics.Image;
+import org.eclipse.swt.graphics.ImageGcDrawer;
 import org.eclipse.swt.graphics.RGB;
 import org.eclipse.swt.graphics.Rectangle;
 import org.eclipse.swt.layout.FillLayout;
@@ -333,10 +334,6 @@ private class PresentationLabelProvider extends LabelProvider implements IFontPr
 
 		private final HashMap<Color, Image> images = new HashMap<>();
 
-		private int imageSize = -1;
-
-		private int usableImageSize = -1;
-
 		private static final int REFRESH_INTERVAL_IN_MS = 300;
 
 		private final Runnable updateControlsAndRefreshTreeRunnable = () -> {
@@ -440,25 +437,20 @@ public Image getImage(Object element) {
 					c = display.getSystemColor(SWT.COLOR_WHITE);
 					foregroundColor = display.getSystemColor(DEFINITION_NOT_AVAIL_COLOR);
 				}
+				final Color gcBackgroundcolor = c;
+				final Color gcForegroundColor = foregroundColor;
 				Image image = images.get(c);
 				if (image == null) {
-					ensureImageSize();
-					image = new Image(display, imageSize, imageSize);
-
-					GC gc = new GC(image);
-					gc.setBackground(tree.getViewer().getControl().getBackground());
-					gc.setForeground(tree.getViewer().getControl().getBackground());
-					gc.drawRectangle(0, 0, imageSize - 1, imageSize - 1);
-
-					gc.setForeground(foregroundColor);
-					gc.setBackground(c);
-
-					int offset = (imageSize - usableImageSize) / 2;
-					gc.drawRectangle(offset, offset, usableImageSize - offset, usableImageSize - offset);
-					gc.fillRectangle(offset + 1, offset + 1, usableImageSize - offset - 1,
-							usableImageSize - offset - 1);
-					gc.dispose();
-
+					ImageGcDrawer imageGcDrawer = (gc, imageWidth, imageHeight) -> {
+						gc.setForeground(tree.getViewer().getControl().getBackground());
+						gc.drawRectangle(0, 0, imageWidth - 1, imageHeight - 1);
+						
+						gc.setForeground(gcForegroundColor);
+						gc.setBackground(gcBackgroundcolor);
+						gc.fillRectangle(2, 2, 11, 11);
+						gc.drawRectangle(2, 2, 11, 11);
+					};
+					image = new Image(display, imageGcDrawer, 16, 16);
 					images.put(c, image);
 				}
 				return image;
@@ -470,13 +462,6 @@ public Image getImage(Object element) {
 			}
 		}
 
-		private void ensureImageSize() {
-			if (imageSize == -1) {
-				imageSize = tree.getViewer().getTree().getItemHeight();
-				usableImageSize = Math.max(1, imageSize - 4);
-			}
-		}
-
 		@Override
 		public String getText(Object element) {
 			if (element instanceof IHierarchalThemeElementDefinition themeElement) {