simgear/screen/video-encoder*: build fixes for ffmpeg code on Windows.

simgear/screen/video-encoder-internal.hxx:
    Fixed initialisation of AVRational instance to work on Windows.
simgear/screen/video-encoder.cxx:
    av_log(): vasprintf() is not available on Windows so use fixed buffer on
    Windows.

simgear/screen/video-encoder-internal.hxx

@@ -279,7 +279,7 @@ struct FfmpegEncoder
             /* Resolution must be a multiple of two. */
             m_codec_context->width = width / 2 * 2;
             m_codec_context->height = height / 2 * 2;
-            m_codec_context->time_base = (AVRational){ 1, 60 };
+            m_codec_context->time_base = AVRational{ 1, 60 };
             //m_codec_context->gop_size = 12; /* emit one intra m_frame every twelve frames at most */
             m_codec_context->pix_fmt = AV_PIX_FMT_YUV420P;
             /* Some formats want m_stream headers to be separate. */

simgear/screen/video-encoder.cxx

@@ -170,10 +170,18 @@ static void av_log(void* /*avcl*/, int level, const char* format, va_list va)
     if (level < 44) sglevel = SG_DEBUG;
     if (level < 54) sglevel = SG_BULK;
     else sglevel = SG_BULK;
-    char* message = nullptr;
+    #ifdef _WIN32
-    vasprintf(&message, format, va);
+        /* Windows does not have vasprintf(). */
+        char message[200];
+        vsnprintf(message, sizeof(message), format, va);
+    #else
+        char* message = nullptr;
+        vasprintf(&message, format, va);
+    #endif
     SG_LOG(SG_VIEW, sglevel, "level=" << level << ": " << message);
-    free(message);
+    #ifndef _WIN32
+        free(message);
+    #endif
 }
 
 VideoEncoder::VideoEncoder(