Add min-icon-size option to enforce minimum icon dimensions

[arebaka]

This PR adds support for a new configuration option and command-line flag: min-icon-size. This setting ensures that icons in notifications are not rendered too small, especially in cases where the icon file (e.g. .svg or low-res .png) is smaller than desired.

Previously, icons were rendered at their native size. This often led to extremely small icons in notifications (e.g. 16x16px icons on high-DPI screens), making them barely visible. Now, Mako will check the dimensions of the loaded icon and, if the largest side is smaller than min-icon-size, it will reload the image using gdk_pixbuf_new_from_file_at_scale() to upscale it to the minimum size while maintaining aspect ratio.

Was:

Became:

Changes:

• New min-icon-size configuration key
  Example:
  min-icon-size = 32

• New command-line override: --min-icon-size=N

• Scaling logic:
  After loading the icon image (via gdk_pixbuf_new_from_file()), check if its width or height is less than min-icon-size.
  If so, reload the image using gdk_pixbuf_new_from_file_at_scale() with the minimum dimension.
  Aspect ratio is preserved.

Benefits:

• Improves visibility and aesthetics of notifications using small icons.
• Particularly useful for .svg icons or legacy bitmap icons at low resolutions.
• Works seamlessly with both old icon-path logic and new icon-theme lookup Use libsfdo-based symbolic/scalable/SVG icon lookup #584 (if present).

Compatibility:

• The new min-icon-size option is optional.
• If unset, Mako retains its original icon rendering behavior.
• Fully backward compatible with existing configurations.

Testing:

• Verified behavior with small PNG icons (16x16) and scalable SVG icons.
• Confirmed that large icons are unaffected.
• Checked integration with both icon lookup paths (libsfdo and legacy glob).

Closes #464
CC: @arebaka

[grolongo]

Also useful for a color picker script. I have one that selects a 1x1 pixel on the screen and sends a notification showing the hex color code with the actual color as an icon.

This is how it shows on dunst:

and mako: