This is a partial revert of rBfbc65c6b28fb4cec4ad67ba0345cd045914af20e.
I wonder if the imbuf_offscreen functions should still use _eval for
their calculations since you may have a new depsgraph for rendering.
But for now I only left ob_eval for the modifiers.
The original issue is that we were changing the camera shiftx
temporarily for the stereoscopic calculation. However we are using the
evaluated object when calculating the projection matrix.
Note: Camera framing drawing for stereo still seems to be broken.
But the viewport itself is now correct.