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/weston.ini b/weston.ini
index 3fda31b..99d24dd 100644
--- a/weston.ini
+++ b/weston.ini
@@ -37,11 +37,14 @@
 #[output]
 #name=LVDS1
 #mode=1680x1050
+#transform=90
 
 #[output]
 #name=VGA1
 #mode=173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync
+#transform=flipped
 
 #[output]
 #name=X1
 #mode=1024x768
+#transform=flipped-270