libobs: Add process piping functions

This adds functions for piping a command line program's stdin or stdout.
Note however that this is unidirectional only.

This will be especially useful later on when implementing MP4 output,
because MP4 output has to be piped to prevent unexpected program
termination from corrupting the file.
libobs: Fix windows warnings with C library macro

The macro INFINITY apparently will trigger a warning with microsoft's
compiler despite being a C standard library macro.
libobs: Fix double -> float conversion warning
libobs: Fix warnings assigning values to vec3

Because a vec3 structure can contain a __m128 variable and not the
expected three floats x, y, and z, you must use vec3_set when
setting a value for a vec3 structure to ensure that it uses the proper
intrinsics internally if necessary.
libobs: Fix microsoft compiler issue

In older versions of visual studio 2013 microsoft's WORTHLESS C compiler
has a bug where it will, almost at random, not be able to handle having
variables declared in the middle of a function and give the warning:
"illegal use of this type as an expression".  It was fixed in recent
VS2013 updates, but I'm not about to force everyone to update to it.
libobs: Add volume meter object

This adds a volume meter object to libobs that can be used by the GUI
or plugins to convert the raw audio level data from sources to values
that can easily be used to display the audio data.
The volume meter object will use the same mapping functions as the
fader object to map dB levels to a scale.
libobs: Add IEC 60-268-18 compliant fader type

This adds a new fader type that implements a position/dB mapping
as specified in IEC 60-268-18.
libobs: Add Logarithmic fader type

This adds the conversion function used by the old obs based on a
logarithmic scale.
obs/libobs: Use new volume meter api

This replaces the old code for the audio meter that was using
calculations in two different places with the new audio meter api.
The source signal will now emit simple levels instead of dB values,
in order to avoid dB conversion and calculation in the source.
The GUI on the other hand now expects simple position values from
the volume meter api with no knowledge about dB calculus either.
That way all code that handles those conversions is in one place,
with the option to easily add new mappings that can be used
everywhere.