Fix bug with automatic video scaling

This is sort of hard to explain:  the scale_video_output function was
overwriting the current frame.  If scaling was disabled, it would do
nothing, and return success, and all would be well.  If it was enabled,
it would then call the scaler, and then replace the contents of the
'data' function parameter with the scaled frame data.  The problem with
this is that I was passing video_output::cur_frame directly, which
overwrites its previous value with the scaled frame data.  Then if
cur_frame was not updated on time, it would end up trying to scale the
previously scaled image, if that makes sense.  it would call the video
scaler with the same from for both the source and destination.

So the simple fix was to simply use a local variable and pass that in as
a parameter to prevent this bug from occurring.
video-io: Add function to just get video format
Add API functions for output/encoder scaling

API functions added:
-----------------------------------------------
obs_output_set_preferred_size
obs_output_get_width
obs_output_get_height
obs_encoder_set_scaled_size
obs_encoder_get_width
obs_encoder_get_height

These functions allow for easier means of setting a custom resolution on
an output or encoder.

If an output uses an encoder and you set the preferred width/height
using the output, then the output will attempt to set the scaled
width/height for the encoder it's currently using.

Outputs and encoders now should use these functions to determine the
width/height of the raw frame data instead of using the video-io
functions.
Plugins: Use new encoder/output size functions