added volume rendering to run on GPU

examples_cuda/volume_rendering/volume.ispc

@@ -31,6 +31,17 @@
    SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.  
 */
 
+#ifdef __NVPTX__
+#warning "emitting DEVICE code"
+#define taskIndex blockIndex0()
+#define taskCount blockCount0()
+#define programIndex laneIndex()
+#define programCount warpSize()
+#else
+#warning "emitting HOST code"
+#endif
+
+
 typedef float<3> float3;
 
 struct Ray {
@@ -236,7 +247,7 @@ raymarch(uniform float density[], uniform int nVoxels[3], Ray ray) {
         // terminate once attenuation is high
         float atten = exp(-tau);
         if (atten < .005)
-            cbreak;
+            break;
 
         // direct lighting
         float Li = lightIntensity / distanceSquared(lightPos, pos) *