Add animated level parameter to brightness

Tested using flowblade, both old and new styles of settinh brightness level work here.
- old "start", "end" is used until a value is set to new "level" property
- calculating "start","end" level was moved to method "filter_get_image" and using MLT_FRAME_PROPERTIES was dropped
- "this" was changed to "frame" and "filter" where appropriate