Fix landscape appearance bug in multi-monitor setup

In a multi-monitor setup, the keyboard didn't appear correctly since
commit 9cc8931b46 . This affected only
landscape mode and only on multi-monitor setups where wvkbd is launched
on anything but the first output (and perhaps even only in
multi-monitor setups where one is portrait mode and one landscape like
mine, I'm not sure).

In the mentioned commit Willow commented:

> On situation where multiple screens are present, it will eventually need
> a second loop (roundtrip)

It seems that second loop never really made it all the way to drawing.
Adding an explicit drwsurf_attach() unconditionally in
layer_surface_configure fixes this.

I'm not sure if this is the best way but this fixes this bug, and as far
as I know doesn't cause extra redraws/resizes in the normal
single-monitor scenario.

Signed-off-by: Maarten van Gompel <proycon@anaproy.nl>

keyboard.h

@@ -111,6 +111,7 @@ struct kbd {
 	size_t last_abc_index; //the layer index of the last alphabetical layout
 
 	struct layout *layouts;
+	struct Output *output; //only used to keep track of landscape flipping, never dereferenced
 	enum layout_id *layers;
 	enum layout_id *landscape_layers;