The initial DTS for non-fractional framerates was being incorrectly
calculated.  It assumed that the time base was in frames when it was
not.

Closes obsproject/obs-studio#1857

This reverts commit adf3337d.
Currently causing timestamp issues resulting in FPS issues.

SEI was using the wrong variable for its size.

Prevents a crash when using the new lookahead feature, presumably
because those textures are in use when the EOS is sent.

According to nvidia's documentation using b-frames as reference results in
a slight quality improvement with no/negligible performance impact and is
recommended to be enabled by default when using multiple b-frames.

Because the new NVENC implementation requires texture sharing, if the
user chooses to use a secondary GPU, fall back to the old implementation
instead.

Adds a texture-based NVENC implementation which passes OBS NV12 output
textures directly to NVENC without downloading them off of the GPU,
increasing NVENC performance by a significant margin.

If NV12 textures are unavailable or the new encoder fails to initialize
for whatever reason, it will fall back to the FFmpeg NVENC
implementation safely.