lavd/opengl: select the GL context before drawing

Since “2d924b3a63 fftools/ffmpeg: move each muxer to a separate thread”,
opengl_write_packet() is called from a different thread than
opengl_write_header() and would nothing for lack of a selected context.

libavdevice/opengl_enc.c

@@ -1200,6 +1200,10 @@ static int opengl_draw(AVFormatContext *h, void *input, int repaint, int is_pkt)
     const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pix_fmt);
     int ret;
 
+    /* At this point, opengl->glcontext implies opengl->glcontext */
+    if (opengl->glcontext)
+        SDL_GL_MakeCurrent(opengl->window, opengl->glcontext);
+
 #if CONFIG_SDL2
     if (!opengl->no_window && (ret = opengl_sdl_process_events(h)) < 0)
         goto fail;