prefabs/mesh: make texture parameter optional

[meszaros-lajos-gyorgy]

If omitted it should substitute in Texture.missingTexture, so prototyping shapes could be done faster.

// current way of doing it
const mesh = createPlaneMesh({ size: 1000, texture: Texture.missingTexture })

// proposed new way
const mesh = createPlaneMesh({ size: 1000 })

Other prefabs should have the same feature added as well.

[meszaros-lajos-gyorgy]

implemented in 7e7c75f and released as v21.0.0-alpha.26