don't clear the framebuffer when the framebuffer is not used

when areas of the FB are undefined (transparent windows on top of
nothing), we clear those areas before composition.
however, it makes no sense to do this when the FB is not in use
(case where hwc handles all layers)

Bug: 5360529
Change-Id: I761a7d3664228ebbee0944aca8b0cc278fc94c9c

Author: pixelflinger

services/surfaceflinger/SurfaceFlinger.cpp

@@ -986,19 +986,20 @@ void SurfaceFlinger::setupHardwareComposer(Region& dirtyInOut)
 
 void SurfaceFlinger::composeSurfaces(const Region& dirty)
 {
-    if (UNLIKELY(!mWormholeRegion.isEmpty())) {
+    const DisplayHardware& hw(graphicPlane(0).displayHardware());
+    HWComposer& hwc(hw.getHwComposer());
+
+    const size_t fbLayerCount = hwc.getLayerCount(HWC_FRAMEBUFFER);
+    if (UNLIKELY(fbLayerCount && !mWormholeRegion.isEmpty())) {
         // should never happen unless the window manager has a bug
         // draw something...
         drawWormhole();
     }
 
-    const DisplayHardware& hw(graphicPlane(0).displayHardware());
-    HWComposer& hwc(hw.getHwComposer());
-    hwc_layer_t* const cur(hwc.getLayers());
-
     /*
      * and then, render the layers targeted at the framebuffer
      */
+    hwc_layer_t* const cur(hwc.getLayers());
     const Vector< sp<LayerBase> >& layers(mVisibleLayersSortedByZ);
     size_t count = layers.size();
     for (size_t i=0 ; i<count ; i++) {