Add plane format in weston-debug

This adds support in weston-debug to display the pixel format of the framebuffer attached on the HW plane. It touches both legacy and atomic paths.

While at it, discovered that in some cases (specifically when view being is fully opaque) we wrongly displayed the the client pixel format buffer. This takes that into (consideration), and it also displays the original pixel format and adds a few words about the opaque substitute (as documentation).

This also also prints the DRM FB backing the view when using HW only mode of compositing.