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/volume.vcxproj

@@ -143,7 +143,7 @@
   <ItemGroup>
     <ClCompile Include="volume.cpp" />
     <ClCompile Include="volume_serial.cpp" />
-    <ClCompile Include="tasks_concrt.cpp" />
+    <ClCompile Include="../tasks_concrt.cpp" />
   </ItemGroup>
   <ItemGroup>
     <CustomBuild Include="volume.ispc">