Add support for shared textures to graphics API
Do not init default_rect effect if not on mac
Free graphics subsystem if graphics fails to load

The graphics subsystem was not being freed here, for example if a
required effect failed to compile it would still successfully have the
graphics subsystem sans required effect.  The graphics subsystem should
be completely shut down if required libobs effects fail to compile.
Initialize default_rect_effect only with OpenGL

Instead of limiting this to an apple-specific define, just limit it to
OpenGL -- because this particular effect file cannot be used with
Direct3D.
Disable blending for colorspace conversion and subsampling only

This should fix blending issues on stream/recording that weren't visible
in the preview