lavc: eliminate tb_unreliable()

If framerate estimation is enabled, it makes sense to always apply it,
instead of limiting it to a few specific cases.
avformat/utils: add gif to tb_unreliable()

This improves the used timebases when gif is the source

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/utils: prevent r frame rate from being set larger than 1/tb

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '1eb57e1d9b59db0aa63348c21bf3290bd3f5efcb'

* commit '1eb57e1d9b59db0aa63348c21bf3290bd3f5efcb':
  lavc: eliminate tb_unreliable()

Conflicts:
	libavformat/utils.c

tb_unreliable() is used in several other parts of libavformat and thus isnt removed
The remaining change is a removial of a call to tb_unreliable() which would cause
a practically unconditional delay of about 1 second in startup for live streams.
I will implement what the commit message claims the change should do in the next
commit

This is thus, not merged

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avformat/utils: Ensure that average fps is probed if requested by the user even if tb_unreliable() is 0

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/utils: Set the average frame rate from the r_frame_rate if the stream appears to be cfr

This permits the average fps to be set in more cases

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>