Update semantics for projections with a mesh for each eye.

[acolwell]

• Replaced "stereo layout" with "stereo mode" so use a single term for the same concept.
• Removed language that said to ignore stereo mode if two meshes were present.
• Added new stereo mode to explicitly signal stereo content with two meshes that shouldn't have coordinate adjustments applied.
• Clarified that the existing StereoMode element is used to signal stereo mode information for WebM/Matroska