Blender Git Loki

Fix T54395: Original image size set incorrectly

`SequenceElement` type `orig_height` and `orig_width` members were
set to incorrect size when using proxies and not set when strip was
added which caused value to be unset.

Since now image dimensions must be read when strip is created,
these members can be initialized. When proxies are used, do not set
original size since it is not guaranteed, that proxies are exact size.

These values are not guaranteed to be up to date or exact. They should
be used for strictly informative purposes.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D6506