libdeng2|Widgets: RuleRectangle size and RootWidget size

The RootWidget's size now correctly uses the width and height outputs
of its rule rectangle.
UI|Client: Working on the busy mode transition

Todo: To make this work correctly, the untransformed UI content needs
to be frozen at the window level.
Merge branch 'oculus-rift' of github.com:skyjake/Doomsday-Engine into oculus-rift
Merge branch 'oculus-rift' of github.com:skyjake/Doomsday-Engine into oculus-rift
Merge branch 'master' into oculus-rift
Busy Mode: Drawing game widgets to texture for busy transition

Instead of grabbing a screenshot, which unavoidably is already
affected by VR transforms, we will now render the game widgets to
a texture for the busy transition.

This is an imperfect solution:
- The rest of the UI (taskbar, etc.) disappears during busy mode
  as it is not included in the texture.
- When starting a game from the menu, the game plugin has already
  entered a transitional state and drawing the frame at bust mode
  start produces a black frame.

Todo: Get rid of the separate busy mode widget tree. Instead,
use BusyWidget to replace GameWidget and GameUIWidget during busy
mode (drawing the offscreen frame there), while letting all the
other widgets be present (although input should be mostly disabled).
Refactor|libdeng2|Widgets: Checking for behavior flags in ancestors

Both the ‘Hidden’ and ‘Disabled’ flags are inherited from parents.
UI|Client: Buttons and line editors can be disabled
UI|Busy Mode|Client: Removed additional UI root for busy mode

Busy mode now uses the same widget tree as the normal UI, meaning
that the task bar, notifications, and other game-unrelated widgets
can remain present (if not interactive) also during busy mode.

Todo: The transition screenshot is currently captured always when
starting busy mode, although this is no longer the suitable time.
Instead, the game plugin must notify the engine of a suitable
“UI freeze point” that then gets rendered to a texture for possible
use.
Merge branch 'oculus-rift' of github.com:skyjake/Doomsday-Engine into oculus-rift
Merge branch 'oculus-rift' of ssh://Architecture.local/Users/jaakko/Projects/deng into oculus-rift
Merge branch 'oculus-rift' of ssh://github.com/skyjake/Doomsday-Engine into oculus-rift
Busy Mode|API|libcommon: Busy transition frame is rendered rather than grabbed

This commit changes how the “screen capture” is done for busy mode.
Instead of taking a screen shot of everything in the window (which
may contain elements that should not freeze, or may be affected by
a VR transformation), the Game and GameUI widgets are drawn to a
texture.

However, at the time of the busy mode start, the game state
may already have been modified so that redrawing a frame would not
result in the expected “frozen” view. To get around this, at certain
times like just before executing a game action (e.g., start new game),
the transition frame is always rendered, in case a busy mode will
need it.

Todo: Some slight glitches remain…
Merge branch 'oculus-rift' of ssh://github.com/skyjake/Doomsday-Engine into oculus-rift
Fixed|Stereo 3D|qmake: Debug/release build settings

qmake’s own “debug” and “release” CONFIG values don’t work as one
would expect. Always use “deng_debug” instead.