Determine initial wxSpinCtrlDouble precision

For native wxGTK implementation default spin control precision is derived
from the precision of the increment value. Fot the sake of consistency the
same should be done in the generic implementation.

Closes #18764.

include/wx/generic/spinctlg.h

@@ -380,6 +380,7 @@ public:
                 double inc = 1,
                 const wxString& name = wxT("wxSpinCtrlDouble"))
     {
+        DetermineDigits(inc);
         return wxSpinCtrlGenericBase::Create(parent, id, value, pos, size,
                                              style, min, max, initial,
                                              inc, name);
@@ -410,6 +411,7 @@ protected:
 
     virtual bool DoTextToValue(const wxString& text, double *val) wxOVERRIDE;
     virtual wxString DoValueToText(double val) wxOVERRIDE;
+    void DetermineDigits(double inc);
 
     unsigned m_digits;