Option to set subtitle depth for 3D playback

Expected behavior of the wanted feature

There's currently no feature that would allow us to set the depth of the subtitles when rendered in 3D playback mode. The subtitle is duplicated correctly to show up for both images but since it's rendered in the exact same location on both images its depth will be the depth of the screen.

There should be an option that moves the different instances of the rendered subtitles relative to each other on the x axis, thereby moving them forward or backward relative to the screen depth in 3D playback modes. Implementation shouldn't be more difficult than adding the options' value (or some scaled version of it) to the x position of one of the subtitle instances and substracting it from the other. Default value is 0 to result in current behaviour.

Start by locating the subtitle-rendering path used during 3D playback and trace how the two subtitle instances are positioned. The work is done when a configurable depth value offsets the instances in opposite x directions, while the default value preserves current behavior; add or run the relevant playback tests if that path has coverage.