Use internalYToRow() instead of YToRow(true)

No real changes, just be consistent: as we have internalYToRow() macro
which passes true as "clipToMinMax" YToRow() parameter instead of the
default false, use it everywhere instead of using it almost everywhere
and still using YToRow(true) and XToCol(true) explicitly just once each.

Also add a comment explaining the difference between internalAToB() and
public AToB() functions.

src/generic/grid.cpp

@@ -2505,6 +2505,9 @@ void wxGridWindow::OnFocus(wxFocusEvent& event)
         event.Skip();
 }
 
+// Unlike XToCol() and YToRow() these macros always return a valid column/row,
+// so their results don't need to be checked, while the results of the public
+// functions always must be.
 #define internalXToCol(x, gridWindowPtr) XToCol(x, true, gridWindowPtr)
 #define internalYToRow(y, gridWindowPtr) YToRow(y, true, gridWindowPtr)
 
@@ -8035,7 +8038,7 @@ int wxGrid::GetFirstFullyVisibleRow() const
                                          NULL, &y,
                                          m_gridWin);
 
-        row = YToRow(y, true, m_gridWin);
+        row = internalYToRow(y, m_gridWin);
 
         // If the row is not fully visible (if only 2 pixels is hidden
         // the row still looks fully visible).
@@ -8074,7 +8077,7 @@ int wxGrid::GetFirstFullyVisibleColumn() const
                                          &x, NULL,
                                          m_gridWin);
 
-        col = XToCol(x, true, m_gridWin);
+        col = internalXToCol(x, m_gridWin);
 
         // If the column is not fully visible.
         if ( GetColLeft(col) < x )