It is sometimes needed to present only to a part of a window.
Please, if its not so big problem, would you consider adding source point, dest point, width and height among the present parameters (to allow presenting only a subrect)
and remove the requirement that the image should be the same size as the window.
This is not often needed in games, but in cad-type applications (e.g. map editors and other tools used during game production) it is.
Currently in mantle it is impossible for the application even to do that itself without any kind of support from the api.
Since always the entire window is updated on present, it is impossible to keep some of the parts of the window intact (such as tool bars and other editor UI stuff) that should be independent of the mantle's rendering.
Other example is when there are several independent mantle renderers, each needing to present in different parts of the same window.
It may be possible to bypass this problem with some big changes to the render engines, but it may not be very easy.