Avoid warnings regarding bit-field signedness

Up to (including) C++11, integral bitfields of unspecified signedness have
implementation dependent signedness.

Detected by LGTM.com static analyzer.

Pick-to: 5.15
Change-Id: Ibaa0fdc6e443495a3cd40330c4573c9cc6ccdf5b
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qobject_p.h

@@ -152,7 +152,7 @@ public:
         uint id = 0;
         ushort method_offset;
         ushort method_relative;
-        int signal_index : 27; // In signal range (see QObjectPrivate::signalIndex())
+        signed int signal_index : 27; // In signal range (see QObjectPrivate::signalIndex())
         ushort connectionType : 3; // 0 == auto, 1 == direct, 2 == queued, 4 == blocking
         ushort isSlotObject : 1;
         ushort ownArgumentTypes : 1;

src/gui/painting/qdrawhelper_p.h

@@ -349,8 +349,8 @@ struct QSpanData
         ConicalGradient,
         Texture
     } type : 8;
-    int txop : 8;
-    int fast_matrix : 1;
+    signed int txop : 8;
+    uint fast_matrix : 1;
     bool bilinear;
     QImage *tempImage;
     QRgba64 solidColor;

src/gui/text/qfont_p.h

@@ -97,7 +97,7 @@ struct QFontDef
     uint hintingPreference : 2;
     uint ignorePitch : 1;
     uint fixedPitchComputed : 1; // for Mac OS X only
-    int reserved   : 14; // for future extensions
+    uint reserved   : 14; // for future extensions
 
     bool exactMatch(const QFontDef &other) const;
     bool operator==(const QFontDef &other) const

src/gui/text/qtextdocument_p.h

@@ -113,7 +113,7 @@ public:
     mutable QTextLayout *layout;
     mutable QTextBlockUserData *userData;
     mutable int userState;
-    mutable int revision : 31;
+    mutable signed int revision : 31;
     mutable uint hidden : 1;
 };