avdevice/sdl2: add option to set window position

avdevice/sdl2: add option to set window position

Allows arrangement of multiple windows such as:
ffmpeg -re -f lavfi -i mandelbrot -f sdl -window_x 1 -window_y 1 mandelbrot -vf waveform,format=yuv420p -f sdl -window_x 641 -window_y 1 waveform -vf vectorscope,format=yuv420p -f sdl -window_x 1 -window_y 481 vectorscop

Some changes by Marton Balint:
- allow negative position (partially or fully out-of-screen positions seem to
  be sanitized automatically by SDL (or my WM?), so no special handling is
  needed)
- only show window after the position is set
- do not use resizable and borderless flags at the same time, that caused
  issues in ffplay
- add docs

Signed-off-by: Marton Balint <cus@passwd.hu>