Worker: pass input frame rate to FFmpeg when converting frames to video

FFmpeg needs the input frame rate as well, otherwise it'll default to 25
FPS, and mysteriously drop frames when rendering a 24 FPS shot.

internal/worker/command_ffmpeg.go

@@ -214,6 +214,9 @@ func (p *CreateVideoParams) getInputGlob() ([]string, func(), error) {
 	}
 
 	cliArgs := []string{
+		// FFmpeg needs the input frame rate as well, otherwise it'll default to 25
+		// FPS, and mysteriously drop frames when rendering a 24 FPS shot.
+		"-r", strconv.FormatFloat(p.fps, 'f', -1, 64),
 		"-pattern_type", "glob",
 		"-i", crosspath.ToSlash(p.inputGlob),
 	}

internal/worker/command_ffmpeg_test.go

@@ -42,10 +42,11 @@ func TestCmdFramesToVideoSimplePosix(t *testing.T) {
 
 	cliArgs := []string{
 		"-v", "quiet", // exe
-		"-report",                                              // argsBefore
+		"-report",  // argsBefore
+		"-r", "10", // input frame rate
 		"-pattern_type", "glob", "-i", "path/to/renders/*.png", // inputGlob
 		"-c:v", "hevc", "-crf", "31", "-vf", "pad=ceil(iw/2)*2:ceil(ih/2)*2", // args
-		"-r", "10", // fps
+		"-r", "10", // output frame rate
 		"path/to/renders/preview.mkv", // outputFile
 	}
 	mocks.cli.EXPECT().CommandContext(gomock.Any(), "/path/to/ffmpeg", cliArgs).Return(nil)