remove UNKNOWN field, put DEFAULT = 0

After discussion in the sensors modelling group, we concluded that UNKNOWN should not be used here, and DEFAULT should be the first field.
Also: shorten the lines again, restore old documentation comments

see also discussion on the enum guidelines: OpenSimulationInterface/osi-documentation#68

Signed-off-by: RIFJo <jochmann@rt.rif-ev.de>

Author: RIFJo

osi_sensorview.proto

@@ -313,8 +313,10 @@ message CameraSensorView
 
     // Raw image data.
     //
-    // The raw image data in the memory layout specified by the camera sensor
-    // input configuration. The pixel order is specified in CameraSensorViewConfiguration.pixel_order with the default value PIXEL_ORDER_LEFT_RIGHT_TOP_BOTTOM.
+    // The raw image data in the memory layout specified by the camera    
+    // sensor input configuration. The pixel order is specified in 
+    // CameraSensorViewConfiguration.pixel_order with the 
+    // default value PIXEL_ORDER_LEFT_RIGHT_TOP_BOTTOM.
     //
     optional bytes image_data = 2;
 }

osi_sensorviewconfiguration.proto

@@ -742,32 +742,34 @@ message CameraSensorViewConfiguration
     // Pixel layout documents the order of pixels in the \c image_data
     // in CameraSensorView. 
     //
-    // \note this enum does not contain an entry for right-to-left and bottom-to-top, because this is equivalent to a
-    // 180-degree rotation of the default, which should be indicated in the sensor coordinate
+    // \note this enum does not contain an entry to do mirroring upside down
+    // and left-to-right at the same time, because this is equivalent to a
+    // 180-degree rotation, which should be indicated in the sensor coordinate
     // system.
     //
     enum PixelOrder
     {
-        // Pixel order is unknown (must not be used).
+        // Pixel data is not mirrored (Default).
+        // Pixels are ordered left-to-right, top-to-bottom.
         //
-        PIXEL_ORDER_UNKNOWN = 0;
+        PIXEL_ORDER_DEFAULT = 0;
 
         // Known pixel order that is not supported by OSI.
         // Consider proposing an additional format if using
         // \c #PIXEL_ORDER_OTHER.
         //
         PIXEL_ORDER_OTHER = 1;
-        
-        // Pixels are ordered left-to-right, top-to-bottom (default)
-        //
-        PIXEL_ORDER_LEFT_RIGHT_TOP_BOTTOM = 2;
-        
-        // Pixels are ordered right-to-left, top-to-bottom
+                
+        // Pixels are ordered right-to-left, top-to-bottom.
+        // Pixel data is mirrored along the vertical axis.
+        // The image is mirrored left-to-right.
         //
         PIXEL_ORDER_RIGHT_LEFT_TOP_BOTTOM = 3;
 
-        // Pixels are ordered left-to-right, bottom-to-top
-        //
+        // Pixels are ordered left-to-right, bottom-to-top.
+        // Pixel data is mirrored along the horizontal axis.
+        // The image is mirrored top-to-bottom.
+        // 
         PIXEL_ORDER_LEFT_RIGHT_BOTTOM_TOP = 4;
     }