[Win32] Fix transparency when scaling images with transparentPixel

[ShahzaibIbrahim]

Description

Previously, indexed images that used a transparentPixel (e.g. GIFs) could be implicitly promoted to an 8-bit alpha representation when the image was scaled (for example when switching to a 150% DPI monitor). That meant an image created eagerly at 100% would look correct, but after a zoom/monitor change it would be converted to alpha-based transparency; moving back to 100% then revealed a gray background because the transparency semantics had changed.

This change normalizes indexed images that advertise a transparentPixel during initialization: when no alpha is present we convert the indexed image to a direct 32-bit representation (preserving the mask/transparent semantics) rather than leaving conversion to happen lazily during scaling. By doing this in init(Device, ImageData) the transparency model remains consistent across zoom levels and monitor transitions.

Notes:

• Images that already contain true alpha (alpha/alphaData) are not changed and continue to be handled as 32-bit ARGB.
• Direct images (palette.isDirect) are not affected by this change.
• The Windows mask/icon creation path (TRANSPARENCY_MASK) is preserved.

How to test

1. Run the following snippet on 100% zoom monitor

import org.eclipse.swt.*;
import org.eclipse.swt.graphics.*;
import org.eclipse.swt.widgets.*;


public class ButtonWithGifImage {
    public static void main(String[] args) {
    	System.setProperty("swt.autoScale.updateOnRuntime", "true");
        Display display = new Display();
        Shell shell = new Shell(display);
        shell.setText("SWT Button with GIF Image");
        shell.setSize(300, 200);

        Button button = new Button(shell, SWT.PUSH);
        button.setBounds(80, 60, 140, 60);
        button.setText("Click Me");

        Image image = new Image(display,
            ButtonWithGifImage.class.getResourceAsStream("sample.gif"));
        button.setImage(image);

        shell.open();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch())
                display.sleep();
        }

        image.dispose();
        display.dispose();
    }
}

2. Move the window to a 150% DPI monitor (causing scaling) — verify the image still renders correctly (no jagged/incorrect background).
3. Move back to 100% — verify the image still renders correctly and no gray background appears.

Result

Here's the preview how it looked before and after this PR change. 100 -> 150 -> 100

Before:

After

Fixes: #2494

[merks]

Yeah! 🥳

[github-actions]

Test Results

  108 files   -  7    108 suites   - 7   11m 43s ⏱️ +32s
4 490 tests  - 56  4 476 ✅  - 54  13 💤  - 3  1 ❌ +1 
  255 runs   - 56    255 ✅  - 53   0 💤  - 3  0 ❌ ±0 

For more details on these failures, see this check.

Results for commit 587627a. ± Comparison against base commit 6fa4ee7.

This pull request removes 56 tests.

AllWin32Tests ImageWin32Tests ‑ testDisposeDrawnImageBeforeRequestingTargetForOtherZoom
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[2] false
AllWin32Tests ImageWin32Tests ‑ testImageDataForDifferentFractionalZoomsShouldBeDifferent
AllWin32Tests ImageWin32Tests ‑ testImageShouldHaveDimesionAsPerZoomLevel
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[2] false
AllWin32Tests ImageWin32Tests ‑ test_getImageData_fromCopiedImage
AllWin32Tests ImageWin32Tests ‑ test_getImageData_fromImageForImageDataFromImage
AllWin32Tests TestTreeColumn ‑ test_ColumnOrder
…