libobs: Simplify YUV conversion

Currently several shaders need "DrawMatrix" techniques to support the
possibility that the input texture is a "YUV" format. Also, "DrawMatrix"
is overloaded for translation in both directions when it is written for
RGB to "YUV" only.

A cleaner solution is to handle "YUV" to RGB up-front as part of format
conversion, and ensure only RGB inputs reach the other shaders. This is
necessary to someday perform correct scale filtering without the cost of
redundant "YUV" conversions per texture tap.

A necessary prerequisite for this is to add conversion support for
VIDEO_FORMAT_I444, and that is now in place. There was already a hack in
place to cover VIDEO_FORMAT_Y800. All other "YUV" formats already have
conversion functions.

"DrawMatrix" has been removed from shaders that only supported "YUV" to
RGB conversions. It still exists in shaders that perform RGB to "YUV"
conversions, and the implementations have been sanitized accordingly.