API to expose viewer limits to users

[jleibs]

The rerun viewer has a few constants baked into it related to maximium batch sizes for point-clouds, lines, etc.

Users currently have no way to find out about these limits without logging and then seeing whether or not the viewer itself produces an error.

It would be nice if users could query these limits so that they can do their own client-side validation instead of just logging data that will fail to show up. Without an API contract for this users will end up baking in hard-coded limits that could become invalid or inappropriate in future releases.

[SeaOtocinclus]

Thank you for adding this!