skins2: extend xoffset and yoffset parameters

Till now, these two parameters only had a meaning for geographical positioning.
As explained at http://forum.videolan.org/viewtopic.php?f=26&t=111572,
extending xoffset and yoffset for the general case would help a lot skin
developpers easily position an object of unknown size around a given point :
for instance,
    x="50%" xoffset="-50%" would center an object horizontally.
    x="100%" xoffset="-100%" would set an object to the East

This patch doesn't modify the existing behaviour. It just implements a new
feature for the case where it was a silent no op.

x,y, width, height percentages are computed against the parent container
xoffset and yoffset percentages are computed against the current object

TODO: this approach could be generalized to all objects (need to extend the dtd)

modules/gui/skins2/parser/skin_parser.cpp

@@ -959,9 +959,10 @@ void SkinParser::convertPosition( string position, string xOffset,
                           int refWidth, int refHeight, int* p_x, int* p_y )
 {
     int iPosition = getPosition( position );
-    if( iPosition == POS_UNDEF )
-        return;
-
+    if( iPosition != POS_UNDEF )
+    {
+        // compute offset against the parent object size
+        // for backward compatibility
         int i_xOffset = getDimension( xOffset, refWidth );
         int i_yOffset = getDimension( yOffset, refHeight );
         int i_xMargin = getDimension( xMargin, refWidth );
@@ -986,6 +987,17 @@ void SkinParser::convertPosition( string position, string xOffset,
         // add offset
         *p_x += i_xOffset;
         *p_y += i_yOffset;
+    }
+    else
+    {
+        // compute offset against the current object size
+        int i_xOffset = getDimension( xOffset, width );
+        int i_yOffset = getDimension( yOffset, height );
+
+        // add offset
+        *p_x += i_xOffset;
+        *p_y += i_yOffset;
+    }
 }