Add a varying variable `multiview_data` for both surface and
post-processing shaders. Currently this only works as intended in
surface shader, and additional implementation to facilitate it for
post-processing will be following.
The `multiview_data` contains multiview information like whether
multiview is enabled for the current material and what the eye index is
in that case. This variable is globally available including fragment
shaders so that it can be referenced in user shaders in .material files.
Also this allows us to indirectly reference the constant variables such
as `gl_ViewID_OVR` and `gl_ViewIndex` in a platform-agnostic way.
Make the preprocessor `MATERIAL_FEATURE_LEVEL` available for both
surface and post-processing shaders, it used to be only available for
surface shader, and this will be referenced in post-processing shaders
later to support multiview for post-processing.
923ac98ea8