vector layer: fallbacks are not always rendered with the highest quality shading texture

[adam-ce]

steps to reproduce:

1. navigate to a new area (without cached shading textures)
2. the fallback is rendered with a low quality shading texture
3. set max number of geometries to 255 and then back to 0
4. wait for the fallback to be redrawn
5. it has a higher quality now