Addresses T76003. When using VR with Eevee and viewport denoising,
scene geometry could sometimes be occluded for one eye. Solution is
to use a separate GPUViewport/GPUOffscreen for each VR view instead
of reusing a single one for rendering.
Reviewed By: Julian Eisel, Clément Foucault
Differential Revision: http://developer.blender.org/D11858
c41b93bda5