Support dropped/cropped/blank frames

[nosheen-adil]

isxd movies can have invalid frames of the following type: dropped, cropped, blank.

They are not stored on disk, the only indication of them is in the json metadata. Currently this api breaks when trying to access the last frame of a movie with dropped frames because it's not correctly recalculating the "true" frame index on disk.

We need to update the Timing class to have these properties and update the read/write functions to support these invalid frames.