Various improvements to example task systems in examples/.

- Only have a single copy of all of the tasks_*.cpp sample implementations,
  stored in examples/.
- Reduce dynamic storage allocation and locking in task launch code paths.
- Don't have a hard limit of the number of tasks that can be launched on
  Windows (fix issue #85).

examples/volume_rendering/Makefile

@@ -1,18 +1,18 @@
 
 ARCH = $(shell uname)
 
-TASK_CXX=tasks_pthreads.cpp
+TASK_CXX=../tasks_pthreads.cpp
 TASK_LIB=-lpthread
 
 ifeq ($(ARCH), Darwin)
-  TASK_CXX=tasks_gcd.cpp
+  TASK_CXX=../tasks_gcd.cpp
   TASK_LIB=
 endif
 
-TASK_OBJ=$(addprefix objs/, $(TASK_CXX:.cpp=.o))
+TASK_OBJ=$(addprefix objs/, $(subst ../,, $(TASK_CXX:.cpp=.o)))
 
-CXX=g++ -m64
-CXXFLAGS=-Iobjs/ -O3 -Wall
+CXX=g++
+CXXFLAGS=-Iobjs/ -O3 -Wall -m64
 ISPC=ispc
 ISPCFLAGS=-O2 --target=sse4x2 --arch=x86-64
 
@@ -32,6 +32,9 @@ volume: dirs objs/volume.o objs/volume_serial.o objs/volume_ispc.o $(TASK_OBJ)
 objs/%.o: %.cpp
 	$(CXX) $< $(CXXFLAGS) -c -o $@
 
+objs/%.o: ../%.cpp
+	$(CXX) $< $(CXXFLAGS) -c -o $@
+
 objs/volume.o: objs/volume_ispc.h 
 
 objs/%_ispc.h objs/%_ispc.o: %.ispc