UV Editor: Add pixel-based outline preview for padding check (e.g. +4px virtual border)

Description

In texture workflows (especially for games and stylized assets), artists need to ensure that UV shells have enough padding between each other (commonly 2–16 pixels depending on resolution). Currently, Blender offers Pack Islands with Padding, but this only applies to automatic packing. When manually arranging UV shells, there is no way to visually check exact pixel padding.

Proposal

Add an option in the UV Editor to display a virtual outline (expand/contract) around UV shells in pixel units. For example:

• The user enters a value (e.g. +4px).
• The UV Editor draws a visible outline offset by 4 pixels around each shell.
• This outline is not real geometry or UVs, only a display helper.
• This way, artists can visually confirm if their UV shells maintain the correct distance, without needing external tools or scripts.

Why it’s important

• Reduces texture bleeding issues in games and real-time rendering.
• Matches workflows in tools like Substance Painter, which already provide padding preview.
• Helps teams standardize texel density and UV quality control.
• Saves time: no need to run packing repeatedly just to check spacing.

Current limitations in Blender

• UV Editor only shows shell edges (1px wide, scale-dependent).
• No way to preview pixel padding interactively.
• Workarounds require external tools (RizomUV, Substance) or custom scripts.

References / Inspiration

• Substance Painter’s “Padding Preview” option, where the artist can check edge dilation in pixels.
• Photoshop’s “Stroke” effect for visual clarity around shapes.

Suggested implementation

• An overlay toggle in the UV Editor (like “Stretch” or “Face Orientation”).
• User inputs padding value (in pixels).

• Blender converts pixel distance → UV space based on active texture resolution, and draws a virtual outline.

This is a small but high-value feature that would make Blender’s UV Editor much more production-friendly, especially for game artists.