Blender Git Loki

BGE: Save screenshots in a different thread

This patch allows the game engine to keep running while performing things like PNG compression and disk I/O.

As an example, my crowd simulation rasterizer saves a screenshot for every frame. This now takes up 13 msec per frame, which was 31 msec before this patch. Effectively, it allows the simulation to save every frame and still run at 60 FPS.

Reviewers: lordloki, moguri, panzergame

Reviewed By: moguri, panzergame

Projects: #game_engine

Differential Revision: https://developer.blender.org/D1507