GstVideoEncoder could expose a "max-keyframe-distance" property

Submitted by Mathieu Duponchelle @meh

Link to original bug (#751554)

Description

From IRC:

<Mathieu_Du> Hm I've got an interesting issue here, so youtube says "key-int-max" should be half the framerate, I'd like to have that in the preset, seems a bit involved tho
<__tim> you can't really specify that currently
<Mathieu_Du> Yep that's understandable
<Mathieu_Du> Alternate solution would be to have a key-int-max / framerate ratio in x264enc, but that might be a little too specific ?
<Mathieu_Du> (as a new prop)
<__tim> max-keyframe-distance in nanosecs perhaps

[snip]

__tim> the encoder base class can probably "enforce" that even if the encoder supports forcing keyframes

[snip]

<__tim> the encoder can still set key-int-max based on that property
<__tim> the base class handling is just a fallback really

So yep, being able to specify a maximum keyframe distance in nanoseconds would be a valuable addition, and putting that in the baseclass would indeed allow enforcability, what do you folks think ?