Fix QPlatformPrintDevice::supportedPageSizeMatch

First check if the pagesize is one of the known ones before
comparing the page sizes. It's veeeery similar but without
it when running the qprinter tests i get a failure since we
set "Index Card 3x5in" but get back "Photo 3x5in"

Change-Id: I72671231c2abec39e036de829d226784f06e1b41
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/printsupport/kernel/qplatformprintdevice.cpp

@@ -247,6 +247,10 @@ QPageSize QPlatformPrintDevice::supportedPageSize(const QSizeF &size, QPageSize:
 
 QPageSize QPlatformPrintDevice::supportedPageSizeMatch(const QPageSize &pageSize) const
 {
+    // If it's a known page size, just return itself
+    if (m_pageSizes.contains(pageSize))
+        return pageSize;
+
     // Try to find a supported page size based on point size
     for (const QPageSize &ps : m_pageSizes) {
         if (ps.sizePoints() == pageSize.sizePoints())