Replace ARB_separate_shader_objects extension

This replaces the ARB_separate_shader_objects extension with traditional
linked shaders.  I was able to get the existing system to use linked
shaders without having to change any libobs graphics API.

This essentially creates a linked list of shader programs with
references to the shaders they link.  Before draw, it searches that
linked list for a particular pixel/vertex shader pair, and the linked
program associated with it.  If no matching program exists, it creates
the program.
Remove unused variables