OSX and linux build fixes

xs/src/libslic3r/Print.cpp

@@ -1334,7 +1334,8 @@ public:
     }
 
     inline void beginLayer() {
-        layers_rst_.emplace_back(Raster(res_, pxdim_), std::stringstream());
+        layers_rst_.emplace_back();
+        layers_rst_.front().first.reset(res_, pxdim_);
     }
 
     inline void finishLayer(unsigned lyr_id) {

xs/src/libslic3r/Rasterizer/Rasterizer.cpp

@@ -16,7 +16,7 @@
 #include <agg/agg_path_storage.h>
 
 // For png compression
-#ifdef WIN32
+#if defined(WIN32) || defined(__APPLE__ )
 inline char *strerror_r(int errnum, char *buf, size_t buflen) {
     strerror_s(buf, buflen, errnum);
     return buf;