1-bpp BMP export/preview causes pixel shifting/corruption on non-32-bit aligned widths (e.g., 10px)

[nxyd]

Describe the bug

When exporting or previewing an image as a 1-bpp Monochromatic BMP with a custom width that is not a multiple of 32 bits (such as 10px × 16px), the pixel layout becomes corrupted and shifted in the preview window (and potentially in the exported file).

It appears that the DIB scanline padding bytes (4-byte alignment requirement for BMP) are being incorrectly parsed or treated as active pixels during the stride/offset calculation for the next row.

Steps to reproduce the behavior

1. Create a new image with a size of 10 x 16 pixels.
2. Use the Pencil tool (with antialiasing turned off) to draw a clean diagonal line.
3. Go to File -> Save As -> Choose BMP.
4. In the BMP options dialog, select:
   • Color model: Grayscale
   • Depth: 1-bpp
5. Look at the after preview window. The smooth diagonal line becomes jagged, disconnected, and shifted horizontally near the bottom.

Expected behavior

The 1-bpp preview/export should perfectly retain the original pixel connectivity and positioning without any horizontal shifting, regardless of whether the canvas width matches the 4-byte (32-bit) stride alignment.

Environment

• PhotoDemon Version: 2025.12 (as seen in the title bar)
• OS: Windows 11 / Windows 10