Somewhere between versions 0.16 and 0.18 wlroots implemented rendering
code optimizations. One of optimizations was to not call frame callback
for invisible surfaces. This caused bar to not reappear, if it was fully
hidden behind non-transparent window.
This is trivial in the Wayland backend; just instantiate a pixman
pointing to the same mmapped memory as the wayland buffer.
In the XCB backend, we change the implementation slightly; instead of
rendering via a cairo XCB surface backend (to a server side pixmap),
which is then blitted to the window in commit(), we now render to a
client-side pixman pixmap, and blit it using xcb_put_image() in
commit().
All decoration, particle and module interfaces now takes a
pixman_image_t parameter, and all drawing is done using pixman APIs.
The wayland/xcb backends implement a new interface functions,
get_pixman_image(), that should return a pixman image instance that is
suitable for rendering.
In the wayland backend, the image uses the same backing data as the
cairo surface.
In the XCB backend, we create a new image each time, and then blit it
to the cairo surface at commit time.
