Make xrt_hmd_parts->blend_mode an array instead of bitfield

Another quality of life MR, mostly for North Star. For North Star, we need to support opaque and additive blend modes, and specify that additive is preferred. This is because a lot of XR apps still require opaque, and some (like StereoKit) do different things depending on what the preferred mode is. Was not possible with a bitfield, so this MR changes it to an array.