bug(three): createGradient divides by zero when size <= 1

[YayoRazo]

Description

TextureUtils.createGradient() accepts a size parameter (default 256). When size <= 1, the denominators in the gradient calculation become zero, producing Infinity values that propagate through the color lerp and result in NaN pixel values.

• Horizontal/vertical: t = x / (size - 1) → 0 / 0 = NaN
• Radial: t = Math.sqrt(...) / (size / 2) → 0 / 0 = NaN

Steps to reproduce

TextureUtils.createGradient(1) // or size 0
// Result: texture filled with NaN pixels

Expected behavior

Textures with size <= 1 should produce at least a 2x2 gradient, or at minimum not crash with NaN.