Add two options for width and height

[darixsamani]

Description:

Currently, pdf2image-cli only supports controlling output image resolution via the --dpi flag. This works, but it's an indirect way to size images — for many automation and pipeline use cases (thumbnails, fixed-size previews, feeding images into a downstream model with fixed input dimensions, etc.), it would be much simpler to specify the exact pixel dimensions directly.

Proposal

Add two new CLI options:

• --width <PIXELS> — target output image width in pixels
• --height <PIXELS> — target output image height in pixels

Suggested behavior

• If both --width and --height are provided, resize/render the page to exactly those dimensions.
• If only one of the two is provided, the other should be computed automatically to preserve the page's aspect ratio.
• If neither is provided, fall back to the current --dpi-based behavior (no breaking change).
• If --width/--height are used together with --dpi, either the explicit dimensions should take precedence, or the tool should return a clear validation error explaining that the two options are mutually exclusive (whichever the maintainer prefers — happy to open a PR either way).

Example usage

pdf2images --input Fuzzy.pdf --output-dir images --width 1024 --height 768 --format png

# aspect-ratio preserved, height auto-computed
pdf2images --input Fuzzy.pdf --output-dir images --width 800 --format png

Motivation

• More intuitive for users who think in pixels rather than DPI.
• Useful for generating consistently-sized thumbnails/previews across PDFs with different page sizes.
• Keeps the tool convenient for downstream pipelines that expect a known image resolution.