Blender Git Loki

GPencil: Support camera "Frame Selected" and object previews

Using the "Camera Fit Frame to Selected" operator didn't work for Grease Pencil
objects. The same issue caused grease pencil preview thumbnails to be useless
(e.g. when using "Mark Asset" on a Grease Pencil object).
Reason was that there was no logic to handle grease pencil data and its strokes
for the object display-point iterators used for the "Frame Selected" logic.

Addresses T89656.

Reviewed by: Antonio Vazquez, Campbell Barton

Differential Revision: https://developer.blender.org/D11833