#4044 addendum 2: may as well add a geometry buffer for unit models

note: this is *FULLY* compatible with custom Lua UnitRendering
shaders (even easier if some minor framework changes are made)
so the vast majority of deferred rendering operations is now
within Lua's abilities

new configuration-keys to enable/disable G-buffer draw-passes:
	AllowDeferredMapRendering (default true)
	AllowDeferredModelRendering (default true)

new LuaUnsyncedCtrl callouts to enable/disable G-buffer updates:
	Spring.SetDrawGroundDeferred(boolean) -> nil
	Spring.SetDrawModelsDeferred(boolean) -> nil