[upalvstavai]

An empty layered portrait could cache a zero-sized coverage rectangle. The character editor then divided by that empty size while fitting the preview, producing an invalid scale. Because sprite bounds were also measured through global transforms, that invalid preview scale prevented a later inherited texture from recovering the portrait preview.

This change:

• computes sprite bounds relative to the layered portrait from all four transformed corners;
• avoids caching empty coverage so a later inherited texture can populate it;
• guards the character editor against empty or non-finite fit rectangles.

Validation:

• Added regression coverage for an initially empty portrait that later inherits a 24x40 texture
• Added coverage for an invalid root scale while calculating portrait bounds
• gdUnit4: 2/2 tests passed, with no failures, errors, or orphan nodes

Closes #2822