Improve depth perception

[brenocq]

@epezent feedback in #24

Consider adding shading/gradients to plot planes (e.g. darker toward the corners) to add a depth effect

I agree that overall we lack a bit of depth information from the plots. It sounds great to play with gradient colors to bring back some of the depth information. Additionally, we could try to improve depth perception by adding shadows, or some kind of lighting. Even though we can't do per-pixel shading with the ImGuiDrawList, we could potentially do per-vertex shading.

More ideas are more than welcome! I'm still not sure what would be the best strategy and the API for it.