Protect against double-refresh

The wxtimer can fire multiple times and there exists the possiblity for
this to happen before the flags are properly set to prevent it.  This
creates a concurrency mutex to skip the EDA_DRAW_PANEL_GAL refresh if
one is already underway.

Fixes https://gitlab.com/kicad/code/kicad/issues/12094

common/draw_panel_gal.cpp

@@ -174,6 +174,11 @@ void EDA_DRAW_PANEL_GAL::onPaint( wxPaintEvent& WXUNUSED( aEvent ) )
 
 void EDA_DRAW_PANEL_GAL::DoRePaint()
 {
+    if( !m_refreshMutex.try_lock() )
+        return;
+
+    std::lock_guard<std::mutex> lock( m_refreshMutex, std::adopt_lock );
+
     // Repaint the canvas, and fix scrollbar cursors
     // Usually called by a OnPaint event, but because it does not use a wxPaintDC,
     // it can be called outside a wxPaintEvent.

include/class_draw_panel_gal.h

@@ -35,6 +35,7 @@
 #include <math/vector2d.h>
 #include <widgets/msgpanel.h>
 #include <memory>
+#include <mutex>
 
 #include <gal/cursors.h>
 
@@ -253,6 +254,8 @@ protected:
     bool                     m_pendingRefresh;   ///< Is there a redraw event requested?
     wxTimer                  m_refreshTimer;     ///< Timer to prevent too-frequent refreshing
 
+    std::mutex               m_refreshMutex;     ///< Blocks multiple calls to the draw
+
     /// True if GAL is currently redrawing the view
     bool                     m_drawing;