Canvas_View: initialize Computed_Position to No_Position

[fdesp87]

Actually Computed_Position is initialized to (Gdouble'First, Gdouble'First). I believe it should be initialized to No_Position, as it is used when initializing Position a few lines before.

Even if this change is cosmetic, this other is not:
In the procedure Size_Request for Contained_Item_Record, the field Computed_Position is re-initialized to (0.0, 0.0). I believe that this is an error, it should be re-initialized to No_Position.
Currently this error is not manifest because inheritance.