Placeholder textures

## What problem does this solve or what need does it fill?

A placeholder texture fills the need of game developers to:

- Temporarily texture 3D models until the definitive image is made.
- Make obvious that a texture is missing if the placeholder used has standing-out colors (e.g. rainbow or purple-black checkerboard pattern).

## What solution would you like?

- The texture may be procedurally generated or be in a file, depending on what's more convenient for the engine.
- While there is a default placeholder texture, the engine should provide different common patterns, and the user should be able to load or generate their own custom texture.
- The texture should be applied when the asset path does not point to any image.
- The feature may be optimized out in production builds (or not?).

## What alternative(s) have you considered?

Without this feature, a developer must manually generate or download their own placeholder texture, load it in the game, manually check for asset errors, and then apply the texture.

## Additional context

- The idea came out on #4613, where a procedurally generated `Image` is used to texture a set of primitive 3D shapes.