Ignore undefined all-zero alpha data for 32-bit icons without mask

[ShahzaibIbrahim]

I have a png image (icon), which has no maskData and no alpha pixels (that means it is fully opaque):

On Windows, GetDIBits() may return undefined alpha bytes for 32-bit BI_RGB bitmaps, commonly resulting in all-zero alpha even for fully opaque images.

When icon mask data is missing, this caused opaque icons to be incorrectly treated as fully transparent.

Reject all-zero alpha data and only replace empty mask data when the alpha channel contains meaningful (non-opaque, non-zero) values.

How to test

• Run the following snippet on 100% (only reproducible on 100%, otherwise image will be scaled, image semantics will change and we will never run into this issue):

Note: Add the attached image on the same folder as snippet

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

public class ImageTransparencyFix {
    public static void main(String[] args) {
    	System.setProperty("swt.autoScale.updateOnRuntime", "true");
        Display display = new Display();
        Shell shell = new Shell(display);
        shell.setText("Image Transparency Toolbar Demo");
        shell.setSize(300, 200);
        shell.setLayout(new FillLayout());

        Image original = new Image(display, ImageTransparencyFix.class.getResourceAsStream("png-image.png"));

        ImageData data = original.getImageData();
        Image transparentImage = new Image(display, data, data.getTransparencyMask());

        ToolBar imageToolBar = new ToolBar(shell, SWT.FLAT);
        for (int i = 0; i < 3; i++) {
            ToolItem item = new ToolItem(imageToolBar, SWT.PUSH);
            item.setImage(transparentImage);
            item.setToolTipText("Image ToolItem ToolTip " + i);
        }

        // 4) Open UI
        shell.open();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) display.sleep();
        }

        // 5) Dispose resources
        transparentImage.dispose();
        original.dispose();
        display.dispose();
    }
}

• You will see the images on toolbar will be completely transparent.

After fix

[ShahzaibIbrahim]

Drafted because I need suggestion here in the case where, what if the image was made completely transparent by choice? Meaning user define the image to be completely transparent (all 0 alpha pixels). Is this a viable/possible case here? Or that this fix is enough? as we can't know for sure if GetDIBits() is returning 0 as un-defined alpha-byte or was it really 0.

Documentation for GetDIBits -> https://learn.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-getdibits#return-value

[github-actions]

Test Results (win32)

   34 files  ±0     34 suites  ±0   4m 39s ⏱️ +2s
4 636 tests ±0  4 563 ✅ ±0  73 💤 ±0  0 ❌ ±0 
  170 runs  ±0    167 ✅ ±0   3 💤 ±0  0 ❌ ±0 

Results for commit c663e76. ± Comparison against base commit 97a79df.