Stop building mlt_glsl filters and transitions.
Add mlt_glsl_get_effect() and mlt_glsl_add_effect().

These store the movit chain and input on the frame's producer so each
producer can have a unique filter chain.
Refactor movit filters to use mlt_glsl_get/add_effect().
Remove "init glsl" and "start glsl" events from GlslManager.
Remove mlt_glsl and use only GlslManager.
Add Mlt::Deqeue::peek(index).