Suppress various harmless warnings in MinGW build with -Wconversion.

No real changes, simply add casts to make the implicit conversions that g++
warns about when using -Wconversion explicit.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67964 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/mediactrl_am.cpp

@@ -2020,7 +2020,7 @@ wxLongLong wxAMMediaBackend::GetDuration()
 
         case S_OK:
             // outDuration is in seconds, we need milliseconds
-            return outDuration * 1000;
+            return static_cast<wxLongLong>(outDuration * 1000);
     }
 }
 

src/msw/slider.cpp

@@ -602,7 +602,7 @@ wxSize wxSlider::DoGetBestSize() const
             if ( HasFlag(wxSL_MIN_MAX_LABELS) )
                 size.y += labelSize;
             if ( HasFlag(wxSL_VALUE_LABEL) )
-                size.y += labelSize*2.75;
+                size.y += static_cast<int>(labelSize*2.75);
         }
     }
 

src/msw/textctrl.cpp

@@ -1125,7 +1125,7 @@ void wxTextCtrl::AppendText(const wxString& text)
     // don't do this if we're frozen, saves some time
     if ( !IsFrozen() && IsMultiLine() && GetRichVersion() > 1 )
     {
-        ::SendMessage(GetHwnd(), WM_VSCROLL, SB_BOTTOM, NULL);
+        ::SendMessage(GetHwnd(), WM_VSCROLL, SB_BOTTOM, (LPARAM)NULL);
     }
 #endif // wxUSE_RICHEDIT
 }