Patches poppler's pdftocairo to be able to write result to stdout. Tested with pdftocairo 0.22.5.

pdftocairo-stdout.patch

diff --git a/utils/pdftocairo.cc b/utils/pdftocairo.cc
index 192d295..2c4e05a 100644
--- a/utils/pdftocairo.cc
+++ b/utils/pdftocairo.cc
@@ -664,6 +664,9 @@ static GooString *getImageFileName(GooString *outputFileName, int numDigits, int
 {
   char buf[10];
   GooString *imageName = new GooString(outputFileName);
+  if (imageName->cmp("fd://0") == 0) { // stdout
+    return imageName;
+  }
   if (!singleFile) {
     snprintf(buf, sizeof(buf), "-%0*d", numDigits, page);
     imageName->appendf(buf);