random_seed: Support using CryptGenRandom on windows

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/random_seed.c

@@ -23,6 +23,10 @@
 #if HAVE_UNISTD_H
 #include <unistd.h>
 #endif
+#if HAVE_CRYPTGENRANDOM
+#include <windows.h>
+#include <wincrypt.h>
+#endif
 #include <fcntl.h>
 #include <math.h>
 #include <time.h>
@@ -82,6 +86,17 @@ uint32_t av_get_random_seed(void)
 {
     uint32_t seed;
 
+#if HAVE_CRYPTGENRANDOM
+    HCRYPTPROV provider;
+    if (CryptAcquireContext(&provider, NULL, NULL, PROV_RSA_FULL,
+                            CRYPT_VERIFYCONTEXT | CRYPT_SILENT)) {
+        BOOL ret = CryptGenRandom(provider, sizeof(seed), (PBYTE) &seed);
+        CryptReleaseContext(provider, 0);
+        if (ret)
+            return seed;
+    }
+#endif
+
     if (read_random(&seed, "/dev/urandom") == sizeof(seed))
         return seed;
     if (read_random(&seed, "/dev/random")  == sizeof(seed))