ivi-shell: Avoid round-off errors while calculating the surface mask for view

With some layer/surface configurations, observed a pixel line missing from the corner(s) of the rendered surface. Here issue is that the surface mask coordinate generated for the view is one pixel less than expected and is caused by the implicit type casting of floating-point to integer values while calculating the mask.

To avoid the issue, round down left corner and round up right corner coordinates while calculating surface mask.