Fix Issue 870: Crouch toggle not reset when pressing "Fly" button, causes avatar to run/fly downhill.

Reimplements crouch toggle such that crouch is now tracked by the agent object
Fixes crouch being reset when restoring focus to the viewer window, as well.

indra/newview/llappviewer.cpp

@@ -1220,7 +1220,7 @@ bool LLAppViewer::mainLoop()
 				{
 					joystick->scanJoystick();
 					gKeyboard->scanKeyboard();
-					if(isCrouch)
+					if (gAgent.isCrouching())
 					{
 						gAgent.moveUp(-1);
 					}