Change the VTKM_CONT_EXPORT to VTKM_CONT. (Likewise for EXEC and
EXEC_CONT.) Remove the inline from these macros so that they can be
applied to everything, including implementations in a library.
Because inline is not declared in these modifies, you have to add the
keyword to functions and methods where the implementation is not inlined
in the class.
Move some of the management of the width, height, and buffers to the base
Canvas class. Also, when it makes sense, get the width and height from
the rendering system.
Also changed the color buffer to be a Vec so that you don't have to
manage array offsets by hand.
All of these changes snowballed from the observation that the glut
example did not properly enable the depth buffer.
