Add two helper functions for working with aspect ratio embedded into pixbufs. Deprecate x-dpi and y-dpi. Use these functions to allow for custom aspect ratio for pixbufs set with retro_core_view_set_pixbuf(). Update docs.
It's more of an RFC. For Nintendo DS, I need to convince retro-gtk to display the screenshot in the aspect ratio the original game had, because we cannot apply options without booting the core, and the currently loaded game may have different aspect ratio than the preview we want to display if the screen layout is different.