[win32] Add internal strict mode flag for GC

This contribution adds a strict mode flag configurable via system
property to GC the print our errors, if the GC is not configured
correctly for the monitor-specific-scaling

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/GC.java

@@ -102,6 +102,20 @@ public final class GC extends Resource {
 	static final float[] LINE_DASHDOT_ZERO = new float[]{9, 6, 3, 6};
 	static final float[] LINE_DASHDOTDOT_ZERO = new float[]{9, 3, 3, 3, 3, 3};
 
+	/**
+	 * System property to activate a strict mode for GC, which outputs errors for misconfigured GCs.
+	 * <ul>
+	 * <li><b>true</b>: Enables additional checks and prints errors if an invalid configuration is detected.
+	 * This mode should only be used during development.</li>
+	 * <li><b>false</b>: No additional checks are executed.</li>
+	 * </ul>
+	 * <b>Important:</b> This flag is currently only used on Win32. Setting it to
+	 * true on GTK or cocoa wont have any effect.
+	 * <p>
+	 * The current default is "false".
+	 */
+	private static final String GC_STRICT_MODE = "gc.strict";
+
 /**
  * Prevents uninitialized instances from being created outside the package.
  */
@@ -3950,6 +3964,10 @@ void init(Drawable drawable, GCData data, long hDC) {
 }
 
 private static int extractZoom(long hDC) {
+	if (Boolean.getBoolean (GC_STRICT_MODE)) {
+		System.err.println("***WARNING: GC is initialized with a missing zoom. This indicates an "
+				+ "incompatible custom Drawable implementation.");
+	}
 	long hwnd = OS.WindowFromDC(hDC);
 	long parentWindow = OS.GetAncestor(hwnd, OS.GA_ROOT);
 	long monitorParent = OS.MonitorFromWindow(parentWindow, OS.MONITOR_DEFAULTTONEAREST);