This patch adds support to export and import grease pencil in several formats.
Inlude:
* Export SVG
* Export PDF (always from camera view)
* Import SVG
The import and export only support solid colors and not gradients or textures.
Requires libharu and pugixml.
For importing SVG, the NanoSVG lib is used, but this does not require installation (just a .h file embedded in the project folder)
Example of PDF export: https://youtu.be/BMm0KeMJsI4
Reviewed By: #grease_pencil, HooglyBoogly
Maniphest Tasks: T83190, T79875, T83191, T83192
Differential Revision: https://developer.blender.org/D10482
