wxGrid: Fix potential infinite loop with auto-wrap

When using wxGridCellAutoWrapStringRenderer::GetBestWidth() with a cell
containing more newlines than fit, the function never exits because it
keeps on trying to wrap fit into the number of lines based on available
height only.

Fix by also taking into account the number of newlines a cell's text
value has.

Closes #15943.

src/generic/gridctrl.cpp

@@ -546,10 +546,16 @@ wxGridCellAutoWrapStringRenderer::GetBestWidth(wxGrid& grid,
 {
     const int lineHeight = dc.GetCharHeight();
 
-    // Maximal number of lines that fully fit but at least 1.
-    const size_t maxLines = height - AUTOWRAP_Y_MARGIN < lineHeight
-                                ? 1
-                                : (height - AUTOWRAP_Y_MARGIN)/lineHeight;
+    // Base the maximal number of lines either on how many fit or how many
+    // (new)lines the cell's text contains, whichever results in the most lines.
+    //
+    // It's important to take the newlines into account as GetTextLines() splits
+    // based on them and the number of lines returned can never drop below that,
+    // resulting in the while loop below never exiting if there are already more
+    // lines in the text than can fit in the available height.
+    const size_t maxLines = wxMax(
+                              (height - AUTOWRAP_Y_MARGIN)/lineHeight,
+                              1 + grid.GetCellValue(row, col).Freq(wxS('\n')));
 
     // Increase width until all the text fits.
     //