Make gradient fill more interesting and more real:

TRadialGradient and TLinearGradient are two new gradient fill options.
Update a grad2.C tutorial to use TLinearGradient.
Update grad.C tutorial to work with TLinearGradient.
Fix a bug in start/end points calculation for object bounding mode.
Fix (again) a start/end point calculation:

must work with Cocoa coords, not ROOT's coordinates.