Fix last commit regression

It seems it actually needs to be pipe->shape and not pipe->usesShape (i.e. it seems at some point we use pipe->shape with useShape being false)
Otherwise we had a regression on eci_altona-test-suite-v2_technical2_x4.pdf
diff --git a/splash/Splash.cc b/splash/Splash.cc
index cac226d..d8ee640 100644
--- a/splash/Splash.cc
+++ b/splash/Splash.cc
@@ -271,6 +271,7 @@
   // source alpha
   pipe->aInput = aInput;
   pipe->usesShape = usesShape;
+  pipe->shape = 0;
 
   // knockout
   pipe->knockout = knockout;
@@ -467,7 +468,7 @@
     //----- read destination pixel
 
     Guchar *destColorPtr;
-    if (pipe->usesShape && state->blendFunc && pipe->knockout && alpha0Bitmap != NULL) {
+    if (pipe->shape && state->blendFunc && pipe->knockout && alpha0Bitmap != NULL) {
       destColorPtr = alpha0Bitmap->data + (alpha0Y+pipe->y)*alpha0Bitmap->rowSize;
       switch (bitmap->mode) {
         case splashModeMono1: