QPixmapCache: Move qHash(Key) from _p.h to public header

... and make it a hidden friend.

This avoids ODR violations when, because they can't use ours (private,
and not exported), users try to produce their own version of
qHash(Key).

[ChangeLog][QtGui][QPixmapCache] Made the qHash() overload for
QPixmapCache::Key public (was: private) API.

Change-Id: I324ed001c0ae0a251ac2e1d04713013452ffff9c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 7fa3267fda6cff720cca44e829f9a39789031a54)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/image/qpixmapcache.cpp

@@ -218,10 +218,15 @@ QT_BEGIN_INCLUDE_NAMESPACE
 #include "qpixmapcache.moc"
 QT_END_INCLUDE_NAMESPACE
 
-size_t qHash(const QPixmapCache::Key &k, size_t seed)
+/*!
+    size_t QPixmapCache::qHash(const Key &key, size_t seed = 0);
+    \since 6.6
+
+    Returns the hash value for the \a key, using \a seed to seed the calculation.
+*/
+size_t QPixmapCache::Key::hash(size_t seed) const noexcept
 {
-    const auto *keyData = QPMCache::get(k);
-    return qHash(keyData ? keyData->key : 0, seed);
+    return qHash(this->d ? this->d->key : 0, seed);
 }
 
 QPMCache::QPMCache()

src/gui/image/qpixmapcache.h

@@ -31,6 +31,10 @@ public:
         bool isValid() const noexcept;
 
     private:
+        friend size_t qHash(const QPixmapCache::Key &k, size_t seed = 0) noexcept
+        { return k.hash(seed); }
+        size_t hash(size_t seed) const noexcept;
+
         KeyData *d;
         friend class QPMCache;
         friend class QPixmapCache;

src/gui/image/qpixmapcache_p.h

@@ -23,8 +23,6 @@
 
 QT_BEGIN_NAMESPACE
 
-size_t qHash(const QPixmapCache::Key &k, size_t seed = 0);
-
 class QPixmapCache::KeyData
 {
 public: