While Roku might be able to do this through some brightscript transposition 'magic', to do this work for every frame of video might be too demanding for lower end roku hardware. Why not simply reencode the video to fix it, or hold your camera at the correct orientation when filming to begin with?
From Stackoverflow (using FFMPEG)
Rotate 90 clockwise:
ffmpeg -i in.mov -vf "transpose=1" out.mov
For the transpose parameter you can pass:
0 = 90CounterCLockwise and Vertical Flip (default)
1 = 90Clockwise
2 = 90CounterClockwise
3 = 90Clockwise and Vertical Flip
Use -vf "transpose=2,transpose=2" for 180 degrees.
Make sure you use a recent ffmpeg version
Note that this will re-encode the audio and video parts. You can usually copy the audio without touching it, by using -c:a copy. To change the video quality, set the bitrate (for example with -b:v 1M) or have a look at the H.264 encoding guide if you want VBR options.
