Refactor isRenderable detection

[wouterlucas]

✨ What changed? ✨

• Decouple isRenderable from hasRenderableOwner as textures can be loaded asynchronously.
• Refactored the isRenderable detection:
  • If a node has alpha = 0 or is off screen its not renderable
  • If a node has dimensions and color properties, its marked renderable and we assume its a ColorTexture.
  • If a node has a texture, it will be marked as a renderable owner. Which will trigger loading of the texture (if its the first owner)
    • If the texture is loaded, it is marked renderable
• On renderQuads we assume everything has a texture if not we set the defaultTexture for 1x1 sampling. This is implicit but matches the old behaviour.
• CoreTextNode overwrites behaviour and checks for text !== ''.
• Also fixed a bug where the default texture was still loading but already referenced.
• Added a CoreNode unit test to validate this more rapidly.

🔧 Addresses the following
Fixes #477