feat: read custom mesh attribute in user defined extension

[windwhiterain]

I add new function "read_extension" for Reader in mesh mod that comsume a type var T and a path to the accessor index in the json value of extensions, then use the accessor of given index, interpret it as Iterator of T value, return None if no such path exists, panic if the accessor is not exists or can't made into Iterator of T.

[alteous]

Hi, could you run cargo fmt on this and provide some example code (in comments is OK)?

[windwhiterain]

Hi, could you run cargo fmt on this and provide some example code (in comments is OK)?

Hi, thanks for your reply, i will do this in this weekends.