Instead of chaining a bunch of sanity checks together with "&&", let's just have several asserts. That way, when an assert fails, you don't get a monsterous "<huge evil expression>
failed" error, but only the bit you care about, making your life rather a lot easier.

Current implementation returns NaN for some configs of FFMPEG, call default get_fps() instead (as also used by PROP_POS_MSEC)

build for some OCL drivers.
Some OCL vendors treat implicit scalar-vector conversions for
operators as errors when type conversion is required.

QSlider does not have getMinimum/Maximum getters. Probably was not
compile tested.

fixed typo