Implement output transformations.

This patch allows rotation and mirroring outputs for x11 and drm backends.
A new 'transform' key can be set in the [output] section. From the protocol:

"The flipped values correspond to an initial flip around a vertical axis
followed by rotation."

The transform key can be one of the following 8 strings:

normal
90
180
270
flipped
flipped-90
flipped-180
flipped-270
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index 4fc77f1..84eaf6c 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -395,7 +395,8 @@
 	wl_list_insert(&output->base.mode_list, &output->mode.link);
 
 	output->base.current = &output->mode;
-	weston_output_init(&output->base, &c->base, 0, 0, width, height);
+	weston_output_init(&output->base, &c->base, 0, 0, width, height,
+						WL_OUTPUT_TRANSFORM_NORMAL);
 
 	output->base.border.top = c->border.top;
 	output->base.border.bottom = c->border.bottom;