Merge tag 'v0.31.0' into debian

Alpha 31 release
Use persistent buffer mapping for dynamic buffer objects.
Merge branch 'master' into vertexpack
Add files from new assets
Only set the HUD on the initial snapshot

The full function call isn't required. Furthermore, the reported weapon
is incorrect which causes the incorrect HUD to briefly flash on the screen.

Fixes: 408
Always cull using the surface's AABB in R_CullSurface

The AABB is much more accurate than the bounding sphere
Don't needlessly frustum cull in R_CullSurface
Optimize R_SortDrawSurfs
Call Tess_MapVBOs when rendering the sky
Make the parameters to BoxOnPlaneSide const
Ignore mouse up events without a mouse down event
Fixes #447
Delete the old UI vm code
Allow disabling of buffer object extensions

This is useful for testing purposes since these extensions are currently optional
Fix NULL dereference crash
Only drop the mouseup without mousedown event if the key is mouse1
Remove USE_DEFORM_VERTEXES macro.
Remove USE_EYE_OUTSIDE macro.
Merge branch 'master' into vertexpack

Conflicts:
	src/engine/renderer/tr_vbo.cpp
Add cvar to disable GL_ARB_sync.
Fix building on old GLEW.