Fix Windows build break.

Issue #131.

src/pbrt/wavefront/integrator.h

@@ -111,7 +111,7 @@ class WavefrontPathIntegrator {
     void Do(const char *description, F &&func) {
         if (Options->useGPU)
 #ifdef PBRT_BUILD_GPU_RENDERER
-            GPUParallelFor(description, 1, [=] PBRT_GPU(int) { func(); });
+            GPUParallelFor(description, 1, [=] PBRT_GPU(int) mutable { func(); });
 #else
             LOG_FATAL("Options->useGPU was set without PBRT_BUILD_GPU_RENDERER enabled");
 #endif

src/pbrt/wavefront/workqueue.h

@@ -121,7 +121,7 @@ void ForAllQueued(const char *desc, const WorkQueue<WorkItem> *q, int maxQueued,
     if (Options->useGPU) {
         // Launch GPU threads to process _q_ using _func_
 #ifdef PBRT_BUILD_GPU_RENDERER
-        GPUParallelFor(desc, maxQueued, [=] PBRT_GPU(int index) {
+        GPUParallelFor(desc, maxQueued, [=] PBRT_GPU(int index) mutable {
             if (index >= q->Size())
                 return;
             func((*q)[index]);