class TGLViewer: public TVirtualViewer3D, public TGLViewerBase

 Construct the viewer object, with following arguments:
    'pad' - external pad viewer is bound to
    'x', 'y' - initial top left position
    'width', 'height' - initial width/height

gl-embedded viewer's ctor
 Construct the viewer object, with following arguments:
    'pad' - external pad viewer is bound to
    'x', 'y' - initial top left position
    'width', 'height' - initial width/height

 Common initialization.

 Destroy viewer object.

 Entry point for updating viewer contents via VirtualViewer3D
 interface.
 We search and forward the request to appropriate TGLScenePad.
 If it is not found we create a new TGLScenePad so this can
 potentially also be used for registration of new pads.

 Force update of pad-scenes. Eventually this could be generalized
 to all scene-types via a virtual function in TGLSceneBase.

 Resets position/rotation of current camera to default values.

 Setup cameras for current bounding box.

 Perform post scene-build setup.

 Initialise GL state if not already done

 Post request for redraw of viewer at level of detail 'LOD'
 Request is directed via cross thread gVirtualGL object.

 Draw out the the current viewer/scene

 Draw reference marker and coordinate axes.

 Draw camera markup overlay.

 If in debug mode draw camera aids and overall bounding box.

 Perform GL work which must be done before each draw of scene

 Perform GL work which must be done after each draw of scene

 Make GL context current

 Swap GL buffers

 Post request for select draw of viewer, picking objects round the WINDOW
 point (x,y).
 Request is directed via cross thread gVirtualGL object

 Perform GL selection, picking objects overlapping WINDOW
 area described by 'rect'. Return kTRUE if selection should be
 changed, kFALSE otherwise.
 Select lock should already been taken in other thread in
 TGLViewer::ReqSelect().

 Process result from last selection (in fSelRec) and
 extract a new current selection from it.
 Here we only use physical shape.

 Post request for select draw of viewer, picking objects round the WINDOW
 point (x,y).
 Request is directed via cross thread gVirtualGL object

 Perform GL selection, picking overlay objects only.
 Return TRUE if the selected overlay-element has changed.

 Set viewer viewport (window area) with bottom/left at (x,y), with
 dimensions 'width'/'height'

 Return camera reference by type.

 Set current active camera - 'cameraType' one of:
 kCameraPerspX, kCameraPerspY, kCameraPerspZ
 kCameraOrthoXOY, kCameraOrthoXOZ, kCameraOrthoZOY

 Set an orthographic camera to supplied configuration - note this
 does not need to be the current camera - though you will not see
 the effect if it is not.

 'camera' defines the ortho camera - one of kCameraOrthoXOY / XOZ / ZOY
 'left' / 'right' / 'top' / 'bottom' define the WORLD coordinates which
 corresepond with the left/right/top/bottom positions on the GL viewer viewport
 E.g. for kCameraOrthoXOY camera left/right are X world coords,
 top/bottom are Y world coords
 As this is an orthographic camera the other axis (in eye direction) is
 no relevant. The near/far clip planes are set automatically based in scene
 contents

 Set a perspective camera to supplied configuration - note this
 does not need to be the current camera - though you will not see
 the effect if it is not.

 'camera' defines the persp camera - one of kCameraPerspXOZ, kCameraPerspYOZ, kCameraPerspXOY
 'fov' - field of view (lens angle) in degrees (clamped to 0.1 - 170.0)
 'dolly' - distance from 'center'
 'center' - world position from which dolly/hRotate/vRotate are measured
             camera rotates round this, always facing in (in center of viewport)
 'hRotate' - horizontal rotation from initial configuration in degrees
 'hRotate' - vertical rotation from initial configuration in degrees

 Fetch the state of guides (axes & reference markers) into arguments

 Set the state of guides (axes & reference markers) from arguments.

 Draw camera look at and rotation point.

 Return selected physical shape.

 Emit signal indicating selection has changed.

 Emit signal indicating that an overlay drag has finished.

 Calcaulate and return pixel distance to nearest viewer object from
 window location px, py
 This is provided for use when embedding GL viewer into pad

 Process event of type 'event' - one of EEventType types,
 occuring at window location px, py
 This is provided for use when embedding GL viewer into pad

 Handle generic Event_t type 'event' - provided to catch focus changes
 and terminate any interaction in viewer.

 Handle mouse button 'event'.

 Handle mouse double click 'event'.

 Handle configure notify 'event' - a window resize/movement.

 Handle keyboard 'event'.

 Handle mouse motion 'event'.

 Handle window expose 'event' - show.

 Handle window expose 'event' - show.

 Pass viewer for print capture by TGLOutput.

 TVirtualViewer3D interface ... mostly a facade
 Forward to TGLScenePad

 Only implemented because they're abstract ... should throw an
 exception or assert they are not called.

 External GUI component interface

 Draw and selection
 Scene rendering timeouts