vf_drawtext: possible clipping left/right bitmap pitch error

Possible issue when clipping left/right and updating pitch when pixel mode is `MONO` [here](https://github.com/findie/FFmpeg/commit/c28495c55c6fb75a8ad5edc9e92b40761f49c791#diff-3365d6ae06f22ba69bb0eb2a2d647e24R1303) and [here](https://github.com/findie/FFmpeg/commit/c28495c55c6fb75a8ad5edc9e92b40761f49c791#diff-3365d6ae06f22ba69bb0eb2a2d647e24R1326)

Should investigate.