We have GdkDrawingContext, now, which is in charge of creating Cairo
contexts for drawing on a GdkWindow.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

Instead of using gdk_cairo_create(), which we'll soon deprecate.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

They are replaced by the more appropriate gdk_window_begin_draw_frame()
and gdk_window_end_draw_frame() functions.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

Instead of associating the GdkWindow that created the GdkDrawingContext
we can directly bind the Cairo context to the GDK drawing context.

Cairo contexts created via gdk_cairo_create() go back to not having a
GdkWindow associated to them, like they did before we introduced the
gdk_window_begin_draw_frame() API.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

https://bugzilla.gnome.org/show_bug.cgi?id=766675

Instead of giving out Cairo contexts, GdkWindow should provide a
"drawing context", which can then create Cairo contexts on demand; this
allows us to future proof the API for when we're going to use a
different rendering pipeline, like OpenGL.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

Now that GDK has the appropriate API, we can simplify the widget drawing
code into a single function.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

Existing code drawing on a GDK window has to handle the direct drawing
and the buffered drawing by itself, by checking the window type and
whether or not the window is backed by a native windowing surface. After
that, the calling code has to create a Cairo context from the window and
keep an association between the context and the window itself.

This is completely unnecessary: GDK can determine whether or not it
should use a backing store to draw on a GdkWindow as well as create a
Cairo context, and keep track of it.

This allows to simplify the calling code, and enforce some of the
drawing behavior we want to guarantee to users.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

Commit 023f406c9617284ee3b88727df489f20cd066fd6 has a typo that results
in this error:

../../../testsuite/gtk/notify.c: In function 'test_type':
../../../testsuite/gtk/notify.c:679:54: error: expected expression
before ')' token
    (g_str_equal (pspec->name, "max-content-width") ||)
                                                      ^

A wl_buffer has a max size of 4096 bytes, of which 8 are needed for the
header and another 4 for the string argument length (in this case), so
make sure the we only save the first 4083 bytes that are still valid
UTF8.

https://bugzilla.gnome.org/show_bug.cgi?id=767241