Basic assets preview and preview caching

[443eb9]

Preview assets that marked AssetPreview. (might change in the future)

For 2d sprites, this should be a simple sprite 2d.
For meshes, we will render this asset at a low resolution, store the result and display it once needed.
For fonts, render some characters with that font.

TODOs

• Generate preview image for 3d models.
• Cache preview image.
• Regenerate when asset changes, including change during hot reloading.
• Automatically adjust the distance between camera and model according to model scale.