Log an image explicitly as a 3D surface/billboard

[jleibs]

Motivating use-case: I have an image that represents a map. I want to place that map on the "floor" of my 3D space. This is different from a camera projection because I want the image plane to be located at specific geometric location with a specified mapping from pixels to 3d distance.

Example of equivalent functionality in rviz:

This seems like it could mostly be handled with a Rigid3 transform + scale to map from pixel -> spatial coordinates.

[Wumpf]

On the re_renderer side we support arbitrary spanning vectors for textured rectangles. This means that this is mostly a matter of re_viewer and SDK :)