Add background to numbers on meter, IF meter bar and number colour is too similar.

This happens in Hi-Contrast theme.

src/Theme.cpp

@@ -341,6 +341,13 @@ void ThemeBase::RecolourBitmap( int iIndex, wxColour From, wxColour To )
    ReplaceImage( iIndex, pResult.get() );
 }
 
+int ThemeBase::ColourDistance( wxColour & From, wxColour & To ){
+   return 
+      abs( From.Red() - To.Red() )
+      + abs( From.Green() - To.Green() )
+      + abs( From.Blue() - To.Blue() );
+}
+
 // This function coerces a theme to be more like the system colours.
 // Only used for built in themes.  For custom themes a user
 // will choose a better theme for them and just not use a mismatching one.
@@ -353,10 +360,7 @@ void ThemeBase::RecolourTheme()
    wxColour To = wxSystemSettings::GetColour( wxSYS_COLOUR_3DFACE );
 #endif
    // only recolour if recolouring is slight.
-   int d = 
-      abs( From.Red() - To.Red() )
-      + abs( From.Green() - To.Green() )
-      + abs( From.Blue() - To.Blue() );
+   int d = ColourDistance( From, To );
 
    // Don't recolour if difference is too big.
    if( d  > 120 )

src/Theme.h

@@ -121,7 +121,7 @@ public:
    void RecolourBitmap( int iIndex, wxColour From, wxColour To );
    void RecolourTheme();
 
-
+   int ColourDistance( wxColour & From, wxColour & To );
    wxColour & Colour( int iIndex );
    wxBitmap & Bitmap( int iIndex );
    wxImage  & Image( int iIndex );

src/widgets/Meter.cpp

@@ -595,6 +595,14 @@ void Meter::OnPaint(wxPaintEvent & WXUNUSED(event))
    if (mStyle == HorizontalStereoCompact || mStyle == VerticalStereoCompact)
    {
       mRuler.SetTickColour( clrText );
+      // If the text colour is too similar to the meter colour, then we need a background
+      // for the text.  We require a total of at least one full-scale RGB difference.
+      int d = theTheme.ColourDistance( clrText, theTheme.Colour( clrMeterOutputRMSBrush ) );
+      if( d < 256 )
+      {
+         destDC.SetBackgroundMode( wxSOLID );
+         destDC.SetTextBackground( clrBoxFill );
+      }
       mRuler.Draw(destDC);
    }
 #endif