Honor minimum encoder resolution

[AlexKordic]

Fix for TestTranscoder_Portrait test

Add checkEncoderLimits() call at start of Transcode() function that will adjust output resolution to be at least the size of minimum supported resolution.

Hardcoded constraints are taken from 7th Gen NVENC card

var nvidiaCodecSizeLimts = map[VideoCodec]CodingSizeLimit{
    H264: {146, 50, 4096, 4096},
    H265: {132, 40, 8192, 8192},
}

Fix for mistaking landscape for portrait

360p and 240p profiles don't cause the bug, but produce incorrect output size for portrait video inputs.

When input video is in portrait mode and specified output in landscape mode we convert desired output resolution to be in portrait mode.

output resolution	converted
640x360	360x640
426x240	240x426

When output resolution is converted then limits are checked. In case of 144p resolution that causes Invalid argument bug converted resolution is larger than requested to accommodate hardware capabilities.

output resolution	converted
256x144	146x259

How to test

Nvidia card is required.

cd ffmpeg
go test . -v -count 1 -tags nvidia -run TestTranscoder_Portrait
=== RUN   TestTranscoder_Portrait
decoder.c:185] No audio stream found in input
--- PASS: TestTranscoder_Portrait (0.43s)
PASS
ok      github.com/livepeer/lpms/ffmpeg 0.439s