Flesh out and clean up QBackingStore documentation

Some of the functions had wording that suggested people subclass
QBackingStore, probably because the docs came from QWindowSurface
in QWS times.

Clarify the role of the region, window, and offset arguments
to flush(), in the case of flushing child windows.

Document that paintDevice() is only valid after calling beginPaint(),
and should not be cached.

Although our own QRasterWindow, and the rasterwindow example, both
allow backingstores for child windows, and it works in practice on
a few platforms, we keep the documentation stating that they should
only be used for top level windows, until we've verified and
formalized support for this across platforms.

Change-Id: I7b4d6128a0cd2faeb2af9405ac1bcca46440ead1
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 0 insertions, 2 deletions

diff --git a/src/gui/painting/qplatformbackingstore.h b/src/gui/painting/qplatformbackingstore.h
index d1ce67a65d..9956c032a9 100644
--- a/src/gui/painting/qplatformbackingstore.h
+++ b/src/gui/painting/qplatformbackingstore.h
@@ -117,8 +117,6 @@ public:
 
     virtual QPaintDevice *paintDevice() = 0;
 
-    // 'window' can be a child window, in which case 'region' is in child window coordinates and
-    // offset is the (child) window's offset in relation to the window surface.
     virtual void flush(QWindow *window, const QRegion &region, const QPoint &offset) = 0;
 #ifndef QT_NO_OPENGL
     virtual void composeAndFlush(QWindow *window, const QRegion &region, const QPoint &offset,