Fix concurrency bug in image producers.

Reported by Michael Marina.
Fix bug passing serializable data property (SF-202).

Patch by Jakub Ksiezniak.
Move "disable" property doc from service to filter.
Add disable property to mlt_transition.
Do not serialize tractor in/out if unset.
Add mutex locks to mlt_property_anim_get functions.

Patch by Steinar Gunderson.
Convert sRGB to linear in movit.white_balance filter.

Patch by Steinar Gunderson.
Fix title of movit.overlay transition.
Fix deadlock in movit.overlay transition.

Patch by Steinar Gunderson.
Return a sensible size for glsl image types.

Based on patch by Steinar Gunderson.
Fix build on Freetype 2.5.

Reported by Patrick Matthhai/Debian.
Set the return format for default case in color producer.
Fix pbo->size uninitialized.

Patch by Steinar Gunderson.
Convert backtick to $(shell) in Makefiles.
Fix misspelling in consumer_xml.yml.
Propogate service-changed event to cut producer's parent.

Patch by Steinar Gunderson.
Add thread locks around accessing GlslManager resources.
Refactor movit.convert, movit.mix, and movit.overlay.

To use new methods on GlslManager: render_frame_texture() and
render_frame_rgba(). The latter routine was changed to use GL_BGRA in
glReadPixels() to improve performance on more OpenGL implementation (per
Steinar Gunderson's recommendation).
Created a new module to support vid.stab library.