Turn off depth cueing for VR meeting head and hands

[Tom Goddard]

In a VR meeting other people's head and hands vanish due to depth cueing. Should be easy to turn of depth cue on a per-model basis. Maybe this is already done for volumetric rendering.

[Tom Goddard]

Volume rendering turns of SHADER_USE_LIGHTING which eliminates depth cue.

Depth cue is a global setting made by the Render class based on the Lighting settings. Shadows and multishadows are similar global settings. These could be disabled by setting disable shader options. One idea is pass Render.shader(enable_options, disable_options). Currently only enable options are passed. Drawing could have attributes to disable depth cue or shadows and those could be flagged as _effects_shader attributes so the setattr() routine would note the cached Drawing._shader_opt needs updating if those flags change. It is a bit ugly. Another idea would be to require both a global shader depth cue and local shader depth cue bit enabled and just stick with a single shader options flags. The fragment shader would require both bits to enable depth cue. The local bit could be called "allow depth cue".

[Tom Goddard]

With the "allow depth cue" flag it would be desirable if this were combined with the global depth cue flag to produce one depth cue on/off bit for caching shaders. Otherwise different bits could produce the same shader requiring unnecessary shader switches that slow rendering.

[Tom Goddard]

Fixed.

Disabled depth cue on VR face model. This was long overdue, somehow slipped through the cracks.