Blender Git Loki

Cycles X: Implement early cancel in GPU rendering

Allow the GPU rendering to stop rendering before all scheduled paths
has been terminated. This significantly lowers latency of the viewport
navigation.

With some files from the Sprites Fright project the time spend in the
`PathTrace::cancel()` is reduced from 0.6 to below 0.01 sec.

Differential Revision: https://developer.blender.org/D11286