add more concise assert function
add ref to parent tag section for tags
make common operations (such as addition, division,...) return the object instead of a bunch of raw result values.
The previous return value wasn't useful, prevented chaining, added additional overhead and needlessly increased complexity
write new parent refs to ASSLineTagSections constructed from other tag sections or tables and add some additional checks
fix indentation
ASSLineTagSection:getEffectiveTags(): fix a bug that would cause global tags in a first section to be ignored (in favor of style default values) if there were no other sections.
add division operation to tag base class
ASSNumber: hook up metatable events for addition, subtraction, multiplication and division
nil division method added in @4d712f04ef1cca52c7e927ec549a047235804db8 for ASSString and ASSUnknown, which don't have numerical properties
add method for retrieving a table of points (coordinates) to ASSDraw* classes
add method for replacing Tags in a line
ASSAlign: add method for retrieving an offset to positions required to compensate for the alignment
ASSDrawing: add callback method
add method for retrieving the "extreme" points of a drawing, as well as its width and height
add method for trimming and realigning a drawing relative to the coordinate origin (0,0 = left, top)
add method for retrieving a drawing from a clip (either maintaining existing positioning of the line or supplying a new position tag to make the drawing align with the clip)