libmythui: Fix a redraw bug in MythUIType

While evaluating the ATI Catalyst 'TearFree Desktop' option I noticed
that sometimes a keypress was apparently being held up until the time
on the main menu was updated.  Initially I thought that this was a lirc
or fglrx issue, but on investigation it turned out that MythMainWindow::animate
was requesting updates for empty repaint regions.  Delving further
showed that MythUIType::Draw was resetting m_DirtyRegion to empty
even if the clipRect was smaller, thus leaving unpainted regions.

m_DirtyRegion may be extended by HandleMovementPulse, SetRedraw
or SetChildNeedsRedraw etc AFTER GetDirtyArea is called.
So when MythUIType::Draw is called, clipRect may not include the whole
of m_DirtyRegion.

This patch subtracts the Draw clipRect from m_DirtyRegion instead of
simply emtying it.

Fixes #10695

Signed-off-by: Lawrence Rust <lvr@softsystem.co.uk>
Signed-off-by: Stuart Morgan <smorgan@mythtv.org>

mythtv/libs/libmythui/mythuitype.cpp

@@ -478,7 +478,10 @@ void MythUIType::DrawSelf(MythPainter *, int, int, int, QRect)
 void MythUIType::Draw(MythPainter *p, int xoffset, int yoffset, int alphaMod,
                       QRect clipRect)
 {
-    m_DirtyRegion = QRegion(QRect(0, 0, 0, 0));
+    // NB m_DirtyRegion may be extended by HandleMovementPulse, SetRedraw
+    // or SetChildNeedsRedraw etc _AFTER_ GetDirtyArea is called.
+    // So clipRect may not include the whole of m_DirtyRegion
+    m_DirtyRegion -= QRegion(clipRect); // NB Qt >= 4.2
 
     if (!m_Visible || m_Vanished)
         return;
@@ -1406,4 +1409,4 @@ void MythUIType::ConnectDependants(bool recurse)
                 (*it)->ConnectDependants(recurse);
         }
     }
-}
+}