weston_buffer: Change y_inverted to explicit origin enum

y_inverted meant that the buffer's origin was (0,0), and non-inverted
meant that the buffer's origin was (0,height). In practice, every buffer
was 'inverted' into our natural co-ordinate space that we use
everywhere.

Switch to using an explicit origin enum to make this more clear.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/libweston.h

@@ -1191,7 +1191,10 @@ struct weston_buffer {
 
 	int32_t width, height;
 	uint32_t busy_count;
-	int y_inverted;
+	enum {
+		ORIGIN_TOP_LEFT, /* buffer content starts at (0,0) */
+		ORIGIN_BOTTOM_LEFT, /* buffer content starts at (0, height) */
+	} buffer_origin;
 	void *backend_private;
 
 	const struct pixel_format_info *pixel_format;