Fix flickering issues in time boxes and meter panels

Sometimes, resizing the window can cause the project selection, time
toolbars, and/or the input and output VU meter toolbars to flicker. A
simple fix is to set the background style to wxBG_STYLE_PAINT instead of
wxBG_STYLE_SYSTEM because, as the wxWidgets documentation recommends,
"you should change the background style to wxBG_STYLE_PAINT if you define
an EVT_PAINT handler which completely overwrites the window background".
(I'm under the impression that these widgets' paint handlers perform
their own erasing anyways).

Signed-off-by: Avery King <[email protected]>
(cherry picked from commit 4918dfd)

src/widgets/MeterPanel.cpp

@@ -325,6 +325,8 @@ MeterPanel::MeterPanel(TenacityProject *project,
    wxUnusedVar(MicMenu_xpm);
    wxUnusedVar(PrefStyles);
 
+   SetBackgroundStyle(wxBG_STYLE_PAINT);
+
    mStyle = mDesiredStyle;
 
    mIsFocused = false;

src/widgets/NumericTextCtrl.cpp

@@ -1369,6 +1369,8 @@ NumericTextCtrl::NumericTextCtrl(wxWindow *parent, wxWindowID id,
    mAutoPos(options.autoPos)
    , mType(type)
 {
+   SetBackgroundStyle(wxBG_STYLE_PAINT);
+
    mAllowInvalidValue = false;
 
    mDigitBoxW = 11;

src/widgets/NumericTextCtrl.h

@@ -248,7 +248,6 @@ class TENACITY_DLL_API NumericTextCtrl final
    void OnKeyDown(wxKeyEvent &event);
    void OnKeyUp(wxKeyEvent &event);
    void OnMouse(wxMouseEvent &event);
-   void OnErase(wxEraseEvent &event);
    void OnPaint(wxPaintEvent &event);
    void OnFocus(wxFocusEvent &event);
    void OnContext(wxContextMenuEvent &event);