Use malloc to allocate storage for task parameters on Windows.

Fixes bug #55.  A number of tests were crashing on Windows due to the task
launch code using alloca to allocate space for the tasks' parameters.  On
Windows, the stack isn't generally big enough for this to be a good idea.
Also added an alignment parmaeter to ISPCMalloc() to pass the alignment
requirement along.

examples/mandelbrot_tasks/tasks_concrt.cpp

@@ -37,6 +37,7 @@
 #include <windows.h>
 #include <concrt.h>
 using namespace Concurrency;
+#include <stdint.h>
 #include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -45,6 +46,8 @@ using namespace Concurrency;
 extern "C" { 
     void ISPCLaunch(void *f, void *data);
     void ISPCSync();
+    void *ISPCMalloc(int64_t size, int32_t alignment);
+    void ISPCFree(void *ptr);
 }
 
 typedef void (*TaskFuncType)(void *, int, int);
@@ -126,3 +129,13 @@ void ISPCSync() {
 
     taskOffset = 0;
 }
+
+
+void *ISPCMalloc(int64_t size, int32_t alignment) {
+    return _aligned_malloc(size, alignment);
+}
+
+
+void ISPCFree(void *ptr) {
+    _aligned_free(ptr);
+}