Clear the VirtualDevice we create for an impBufferDevice on iOS

Helps rendering of complex SmartArt with shapes a lot, especially such
that include shadows. Exact mechanism unknown. Possibly on other
platforms virtual devices get implicitly "cleared" when created? Sigh,
fun.

SmartArt is still not rendered fully like on desktop (OS X), though.

Change-Id: I1bf92e85b5659ab7f9f4d843b77619de3af36d10

drawinglayer/source/processor2d/vclhelperbufferdevice.cxx

@@ -233,7 +233,15 @@ namespace drawinglayer
 
         if(isVisible())
         {
+#ifdef IOS
+            // Exact mechanism unknown, but for some reason SmartArt
+            // rendering, especially shadows, is broken on iOS unless
+            // we pass 'true' here. Are virtual devices always de
+            // facto cleared when created on other platforms?
+            mpContent = getVDevBuffer().alloc(mrOutDev, maDestPixel.GetSize(), true, 0);
+#else
             mpContent = getVDevBuffer().alloc(mrOutDev, maDestPixel.GetSize(), false, 0);
+#endif
 
             // #i93485# assert when copying from window to VDev is used
             OSL_ENSURE(mrOutDev.GetOutDevType() != OUTDEV_WINDOW,