VR requires glDrawBuffer, which Mesa OpenGL ES does not offer. Default
VR to disabled for OpenGL ES users. Users who want to try it anyway can
still set use_stereo_render=1 in the SCons environment.
Instead of passing a bare `int` named `secret_flag`, define it as an
`enum class : uint8_t` to name the two special values.
Rework the passing of this value, to deal with some confusing
inconsistencies when reading the code.
Before this change:
In D1:
- Multiplayer will always go to the secret level, regardless of which
exit door the player used.
In D2:
- Flying through a D1 secret exit in multiplayer shows the on-HUD error
"Secret Level Teleporter disabled in multiplayer!", and does not exit
the level. This is at best confusing, and at worst dangerous, since
D1 secret exits are only available during the countdown, so the player
has little time to realize that the normal exit must be used instead.
- Like D1, multiplayer will request to go to the secret level regardless
of the exit used. Unlike D1, the caller ignores the flag and always
advances to the next regular level.
After this change:
- No observable differences for the player in-game. The questionable D2
secret exit handling for D1 is retained.
- The code makes clearer that secret exits do not work in D2
multiplayer, by way of `#if defined(DXX_BUILD_DESCENT_I)` guarding the
existence of the parameter and all updates to it.
