Rename stream to sc_demuxer

For consistency with recorder and decoder, name the component which
demuxes a "demuxer".

And add the missing sc_ prefix.

app/meson.build

@@ -9,6 +9,7 @@ src = [
     'src/control_msg.c',
     'src/controller.c',
     'src/decoder.c',
+    'src/demuxer.c',
     'src/device_msg.c',
     'src/icon.c',
     'src/file_pusher.c',
@@ -24,7 +25,6 @@ src = [
     'src/scrcpy.c',
     'src/screen.c',
     'src/server.c',
-    'src/stream.c',
     'src/video_buffer.c',
     'src/util/acksync.c',
     'src/util/file.c',