This algorithm reduces scaling distortion on the center of the image
when scaling from ultrawide to wide.

(Jim: edited effect files to prevent an impact in performance for
standard scaling.  Now effectively generates an extra pixel shader, and
the extra code is only applied to the DrawUndistort technique, while the
original Draw technique is unaffected due to the compiler automatically
removing unused code branches via the hard-coded boolean value)

From jp9000/obs-studio#762

Closes jp9000/obs-studio#526

Refer to https://www.opengl.org/registry/doc/GLSLangSpec.4.10.6.clean.pdf
for a list of current (reserved) keywords.

In the future the shader compiler in libobs-opengl should probably take
care of avoiding those name conflicts (bonus points for transparently
remapping the names of effect parameters)

This adds bicubic and lanczos scaling capability to libobs to improve
scaling quality and sharpness when the output resolution has to be
scaled relative to the base resolution.  Bilinear is also available,
although bilinear has rather poor quality and causes scaling to appear
blurry.

If the output resolution is close to the base resolution, then bilinear
is used instead as an optimization, as there's no need to use these
shaders if scaling is not in use.

The Bicubic and Lanczos effects are also exposed via exported function
to allow the ability to use those shaders in plugin modules if desired.

The API change adds a variable 'scale_type' to the obs_video_info
structure that allows the user interface to choose what type of scaling
filter should be used.