Reduce framerate before downscaling (#7022)

* Reduce framerate before downscaling

It is cheaper to drop frames and downscale those that remain than it is
to downscale all frames and then drop some of them. This is achieved
with the filter chain `-cv fps=FPS,scale=W:H`, and perhaps was the
original intention. The plain `-r` and `-s` flags do not execute in
order though - they each put themselves at the *end* of the filterchain,
so `-r FPS -s WxH` actually applies the scale filter first, and then the
rate filter.

This fix can halve the CPU used by the detect ffmpeg process.

* Bring back hard rate limits

frigate/ffmpeg_presets.py

@@ -107,14 +107,14 @@ def get_selected_gpu(self) -> str:
 }
 
 PRESETS_HW_ACCEL_SCALE = {
-    "preset-rpi-32-h264": "-r {0} -s {1}x{2}",
-    "preset-rpi-64-h264": "-r {0} -s {1}x{2}",
+    "preset-rpi-32-h264": "-r {0} -vf fps={0},scale={1}:{2}",
+    "preset-rpi-64-h264": "-r {0} -vf fps={0},scale={1}:{2}",
     "preset-vaapi": "-r {0} -vf fps={0},scale_vaapi=w={1}:h={2},hwdownload,format=yuv420p",
     "preset-intel-qsv-h264": "-r {0} -vf vpp_qsv=framerate={0}:w={1}:h={2}:format=nv12,hwdownload,format=nv12,format=yuv420p",
     "preset-intel-qsv-h265": "-r {0} -vf vpp_qsv=framerate={0}:w={1}:h={2}:format=nv12,hwdownload,format=nv12,format=yuv420p",
     "preset-nvidia-h264": "-r {0} -vf fps={0},scale_cuda=w={1}:h={2}:format=nv12,hwdownload,format=nv12,format=yuv420p",
     "preset-nvidia-h265": "-r {0} -vf fps={0},scale_cuda=w={1}:h={2}:format=nv12,hwdownload,format=nv12,format=yuv420p",
-    "default": "-r {0} -s {1}x{2}",
+    "default": "-r {0} -vf fps={0},scale={1}:{2}",
 }
 
 PRESETS_HW_ACCEL_ENCODE_BIRDSEYE = {