* [MSPAINT] Fix scrollbar presence on canvas/view fitting
This is a small fix for a scrollbar related edge case functionality exhibited in mspaint (and in a lesser degree also elsewhere in instances of) scrollbar controlled content. As known, the scrollbars presence is given by the size ratio between the accommodating outer frame and the scroll-controlled contents inside it, and it works for the most part. There is, however, a case where the presence of both toolbars is kept even when the contents would fit the frame (if not for the scrollbars themselves that increase the size of the contents' area). Please take the current commit as a demo fix and a provisional correction for mspaint (as I think that the proper fix should reside in common controls).
* [MSPAINT] Fix a small mistake and improve readability in UpdateScrollbox()
Although with no visible impact on testing, I've used a wrong constant related to vertical scrollbox width in one instance (SM_CXHSCROLL instead of SM_CXVSCROLL), and this fixes it. Also, prefer using constants, to improve a bit readability and performance (by avoiding repeated function calls).
