Fix wxMarkupParserAttr's unwinding of colors

Fix the logic for restoring previous span's colors to account for the
possibility of spans that don't change the color, such as in "<i><span
color='red'>...</span></i>foo". Previously, "foo" would always be
rendered black, because unwinding the attributes stack would encounter
an invalid color (which has r=g=b=0) and set it, disregarding and
preexisting attributes.

Because there's code in there that checks whether the attributes are
valid, we need to keep track of both the actually specified attributes
and the currently effective ones, and use the latter for restoration.

src/generic/markuptext.cpp

@@ -156,11 +156,11 @@ public:
 
         // ...but we only need to restore the colours if we had changed them.
         if ( attr.foreground.IsOk() )
-            m_dc.SetTextForeground(GetAttr().foreground);
+            m_dc.SetTextForeground(GetAttr().effectiveForeground);
 
         if ( attr.background.IsOk() )
         {
-            wxColour background = GetAttr().background;
+            wxColour background = GetAttr().effectiveBackground;
             if ( !background.IsOk() )
             {
                 // Invalid background colour indicates that the background