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.