Use a white background for logos in READMEs

[ptrgags]

When you use dark mode, the 3D Tiles logo is hard to read:

the 3D Tiles Next logo has a similar issue.

The easiest thing is to use a logo with a white background in the README.

[javagl]

This is handled now with some GitHub trickery in the main README.md, which says

![](figures/3DTiles_light_color_small.png#gh-dark-mode-only)![](figures/3DTiles_dark_color_small.png#gh-light-mode-only)