hwcomposer: Provide window background without wp_viewporter

[dos1]

Currently, window background is provided only on compositors that support wp_viewporter, by creating a single pixel buffer and scaling it to fill the whole window. Compositors that don't support wp_viewporter, such as Mir, don't get window background (or, to be more exact, only get a single pixel of it in top left corner).

This could be improved by providing a full-sized buffer instead when viewporter support is missing.

See #12 and #33.