Add obs_source_draw helper function

This function simplifies drawing textures for sources in order to help
reduce boilerplate code.  If a source is a custom drawn source, it will
automatically set up the effect to draw the sprite.  If it's not a
custom drawn source, it will simply draw the sprite as per normal.  If
the source uses a specific color matrix, it will also handle that as
well.
Add config_open_string function

This allows opening ini config data from a string.  Before, it was only
possible to load config from a file.
Allow hex numbers to be used in config data

This allows the 0x* format of hexadecimal integers to be used as config
integers.
obs-ffmpeg: If ffaac is used, change cutoff freq

If FFmpeg's experimental aac encoder is used, this changes the cutoff
frequency to better values in order to try to help make up for the
inherent lack of encoder quality a bit.  If FFmpeg is compiled to use
another encoder by default, these settings will not be applied.
text-freetype2: Do not call wcslen in loop expr.

Doing this in the expression will cause it to execute the function every
time the expression is evaluated, which is needless cycles wasted.
Instead, call wcslen before the loops begin.
text-freetype2: Remove null pointer dereference