Fix extreme downscaling of custom stamps

Cody Neimanrequested to merge
xangelix/okular:custom-stamp-size-fix into master

Fix extreme downscaling of custom stamps: Use native resolution for raster and High-DPI for SVG

The current implementation calculates the stamp image size based on the PDF page geometry in Points (1/72 inch). This forces loadStamp to rasterize or downscale the custom stamp image to 72 DPI.

This commit improves setPopplerStampAnnotationCustomImage:

  1. Uses native resolution for raster images (PNG/JPG) to prevent downscaling.
  2. Uses 288 DPI (4x scale) for SVGs to ensure crisp rendering.
  3. Adds a 4096px clamp for safety.

BUG: 370382 (Using non-default size for custom stamp pixelates the stamp)

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