Update ImageDataProvider contract

Clarify in the contract that ImageDataProvider implementations are
expected
to return linearly scaled ImageData based on the zoom level.

This commit also extends the strict checks for verifying linear scaling
to GTK and Cocoa implementations.

Author: arunjose696

bundles/org.eclipse.swt/Eclipse SWT/common/org/eclipse/swt/graphics/ImageDataProvider.java

@@ -28,16 +28,31 @@ public interface ImageDataProvider {
 
 	/**
 	 * Returns the image data for the given zoom level.
-	 * <p>
-	 * If no image is available for a particular zoom level, this method should
-	 * return <code>null</code>. For <code>zoom == 100</code>, returning
-	 * <code>null</code> is not allowed, and SWT will throw an exception.
+	 *
+	 * <ul>
+     *   <li>
+     *     If no image is available for a particular zoom level, this method should
+     *     return <code>null</code>. For <code>zoom == 100</code>, returning
+     *     <code>null</code> is not allowed, and SWT will throw an exception.
+     *   </li>
+     *   <li>
+     *     Implementations are expected to return {@link ImageData} that is
+     *     <b>linearly scaled</b> with respect to the zoom level.
+     *     For example, if <code>getImageData(100)</code> returns an image of
+     *     width <code>w</code> and height <code>h</code>, then
+     *     <code>getImageData(200)</code> must return an {@link ImageData}
+     *     of width <code>2 * w</code> and height <code>2 * h</code>,
+     *     if a non-{@code null} result is returned.
+     *   </li>
+     * </ul>
+	 *
 	 *
 	 * @param zoom
 	 *            The zoom level in % of the standard resolution (which is 1
 	 *            physical monitor pixel == 1 SWT logical point). Typically 100,
 	 *            150, or 200.
-	 * @return the image data, or <code>null</code> if <code>zoom != 100</code>
+	 * @return the linearly scaled image data for the given zoom level,
+	 *         or <code>null</code> if <code>zoom != 100</code>
 	 *         and no image is available for the given zoom level.
 	 * @since 3.104
 	 */