Update grid row/column sizes when switching DPI

Because the size of the text (usually) shown in the grid changes, the
size of its columns/rows should change as well when DPI changes. Scale
them by the DPI ratio if necessary, i.e. if any of them were set to
non-default values to achieve this.

Note that this is not ideal because it can result in rounding errors and
hence in slight change of row/column size when moving the grid window to
another display with a different DPI and then back, but to prevent this
from happening we'd need to store the column/rows sizes in DIPs inside
wxGrid and convert them to actual physical pixels everywhere, which
would require much more changes. Still, if the round trip problems turn
out to be a real problem in practice, we probably will need to do this.

Also note that this doesn't take care of the grid with a native header,
but this will be addressed by upcoming commits.

src/generic/grid.cpp

@@ -5354,9 +5354,50 @@ void wxGrid::OnDPIChanged(wxDPIChangedEvent& event)
 {
     InitPixelFields();
 
+    // If we have any non-default row sizes, we need to scale them (default
+    // ones will be scaled due to the reinitialization of m_defaultRowHeight
+    // inside InitPixelFields() above).
+    if ( !m_rowHeights.empty() )
+    {
+        int total = 0;
+        for ( unsigned i = 0; i < m_rowHeights.size(); ++i )
+        {
+            int height = m_rowHeights[i];
+
+            // Skip hidden rows.
+            if ( height <= 0 )
+                continue;
+
+            height = height * event.GetNewDPI().x / event.GetOldDPI().x;
+            total += height;
+
+            m_rowHeights[i] = height;
+            m_rowBottoms[i] = total;
+        }
+    }
+
+    // Similarly for columns.
+    if ( !m_colWidths.empty() )
+    {
+        int total = 0;
+        for ( unsigned i = 0; i < m_colWidths.size(); ++i )
+        {
+            int width = m_colWidths[i];
+
+            if ( width <= 0 )
+                continue;
+
+            width = width * event.GetNewDPI().x / event.GetOldDPI().x;
+            total += width;
+
+            m_colWidths[i] = width;
+            m_colRights[i] = total;
+        }
+    }
+
     InvalidateBestSize();
 
-    CalcWindowSizes();
+    CalcDimensions();
 
     Refresh();