Add obs_get_default_rect_effect

This provides a default effect for users of GL_TEXTURE_RECTANGLE/textures
that return true for gs_texture_is_rect
Add mac-syphon plugin

Allows adding Syphon servers as sources, and provides game-capture if
used with SyphonInject (specifically the scripting additions have to be
installed for SyphonInject to work from within OBS)