avformat: implement SChannel SSP TLS protocol

This implementation does not support TLS listen sockets and loading
CA/Certs from files.

The Windows API does not support loading PEM certs, and would either
require a manual loader or instead be limited to loading Windows PFX
certificates

TLS listen sockets would have to be implemented quite separately, as many
of the APIs are different for server-mode (as opposed to client mode).

configure

@@ -281,6 +281,8 @@ External library support:
   --enable-opengl          enable OpenGL rendering [no]
   --enable-openssl         enable openssl, needed for https support
                            if gnutls is not used [no]
+  --disable-schannel       disable SChannel SSP, needed for TLS support on
+                           Windows if openssl and gnutls are not used [autodetect]
   --disable-sdl            disable sdl [autodetect]
   --disable-securetransport disable Secure Transport, needed for TLS support
                            on OSX if openssl and gnutls are not used [autodetect]
@@ -1468,6 +1470,7 @@ EXTERNAL_LIBRARY_LIST="
     opencl
     opengl
     openssl
+    schannel
     sdl
     securetransport
     x11grab
@@ -2763,13 +2766,15 @@ sctp_protocol_deps="struct_sctp_event_subscribe"
 sctp_protocol_select="network"
 srtp_protocol_select="rtp_protocol"
 tcp_protocol_select="network"
-tls_gnutls_protocol_deps="gnutls !tls_securetransport_protocol"
+tls_gnutls_protocol_deps="gnutls !tls_schannel_protocol !tls_securetransport_protocol"
 tls_gnutls_protocol_select="tcp_protocol"
-tls_openssl_protocol_deps="openssl !tls_securetransport_protocol !tls_gnutls_protocol"
+tls_openssl_protocol_deps="openssl !tls_schannel_protocol !tls_securetransport_protocol !tls_gnutls_protocol"
 tls_openssl_protocol_select="tcp_protocol"
+tls_schannel_protocol_deps="schannel"
+tls_schannel_protocol_select="tcp_protocol"
 tls_securetransport_protocol_deps="securetransport"
 tls_securetransport_protocol_select="tcp_protocol"
-tls_protocol_deps_any="tls_securetransport_protocol tls_gnutls_protocol tls_openssl_protocol"
+tls_protocol_deps_any="tls_schannel_protocol tls_securetransport_protocol tls_gnutls_protocol tls_openssl_protocol"
 udp_protocol_select="network"
 udplite_protocol_select="network"
 unix_protocol_deps="sys_un_h"
@@ -5511,6 +5516,9 @@ disabled securetransport || { check_func SecIdentityCreate "-Wl,-framework,CoreF
     check_lib2 "Security/SecureTransport.h Security/Security.h" "SSLCreateContext SecItemImport" "-Wl,-framework,CoreFoundation -Wl,-framework,Security" &&
     enable securetransport; }
 
+disabled schannel || { check_func_headers "windows.h Security.h" InitializeSecurityContext -DSECURITY_WIN32 -lSecur32 &&
+                       enable schannel && add_extralibs -lSecur32; }
+
 makeinfo --version > /dev/null 2>&1 && enable makeinfo  || disable makeinfo
 enabled makeinfo \
     && [ 0$(makeinfo --version | grep "texinfo" | sed 's/.*texinfo[^0-9]*\([0-9]*\)\..*/\1/') -ge 5 ] \

libavformat/Makefile

@@ -544,6 +544,7 @@ OBJS-$(CONFIG_SUBFILE_PROTOCOL)          += subfile.o
 OBJS-$(CONFIG_TCP_PROTOCOL)              += tcp.o
 OBJS-$(CONFIG_TLS_GNUTLS_PROTOCOL)       += tls_gnutls.o tls.o
 OBJS-$(CONFIG_TLS_OPENSSL_PROTOCOL)      += tls_openssl.o tls.o
+OBJS-$(CONFIG_TLS_SCHANNEL_PROTOCOL)     += tls_schannel.o tls.o
 OBJS-$(CONFIG_TLS_SECURETRANSPORT_PROTOCOL) += tls_securetransport.o tls.o
 OBJS-$(CONFIG_UDP_PROTOCOL)              += udp.o
 OBJS-$(CONFIG_UDPLITE_PROTOCOL)          += udp.o

libavformat/allformats.c

@@ -395,6 +395,7 @@ void av_register_all(void)
     REGISTER_PROTOCOL(SRTP,             srtp);
     REGISTER_PROTOCOL(SUBFILE,          subfile);
     REGISTER_PROTOCOL(TCP,              tcp);
+    REGISTER_PROTOCOL(TLS_SCHANNEL,     tls_schannel);
     REGISTER_PROTOCOL(TLS_SECURETRANSPORT, tls_securetransport);
     REGISTER_PROTOCOL(TLS_GNUTLS,       tls_gnutls);
     REGISTER_PROTOCOL(TLS_OPENSSL,      tls_openssl);

libavformat/tls.h

@@ -26,7 +26,7 @@
 #include "url.h"
 #include "libavutil/opt.h"
 
-#define CONFIG_TLS_PROTOCOL (CONFIG_TLS_GNUTLS_PROTOCOL | CONFIG_TLS_OPENSSL_PROTOCOL | CONFIG_TLS_SECURETRANSPORT_PROTOCOL)
+#define CONFIG_TLS_PROTOCOL (CONFIG_TLS_GNUTLS_PROTOCOL | CONFIG_TLS_OPENSSL_PROTOCOL | CONFIG_TLS_SECURETRANSPORT_PROTOCOL | CONFIG_TLS_SCHANNEL_PROTOCOL)
 
 typedef struct TLSShared {
     char *ca_file;