Fix memory leak when opening and closing remux window without remuxing

The remuxer thread was only started if there was an actual remux job,
which resulted in the remuxer thread not being able to call the worker's
destructor (because it wasn't running)
Propagate preview window selection to sources list
Fix wrong selection of items near origin in the preview window

If you have an item with width = 0 and height = 0 close to (0, 0)
clicking anywhere in the preview window would select them
Do not draw selected item outline if selected item is invisible

This should probably be revisited if the preview can be rotated in 3D,
or someone comes up with a good 3D manipulation idea for the preview
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
Log warning when falling back to OpenGL on windows
Add debian packaging files