Blender Git Loki

Fix T73188: RenderResult as Camera BG Image

In blender 2.79 you could use a render result as a camera background
image. This is useful during layout/compositing. During Blender 2.80
development there were 2 issues introduced that removed this feature.

* to receive a render result the image required a lock. This lock wasn't passed and therefore no image was read from the result. Generating an GPUTexture from an Blender image also didn't do the locking.
* the iuser->scene field wasn't set what is required for render results.

This change adds an optional `ibuf` parameter to `GPU_texture_from_blender` that can be passed when available.

Reviewed By: fclem, brecht

Differential Revision: https://developer.blender.org/D6684