new attribute needed to detail exact physical dimensions corresponding to display window

Various packages that reconstruct camera position and orientation from image sequences require their users to enter the position and dimensions of the rectangle on the sensor or film plane that corresponds to the display window. Alternatively the dimensional specification is in pixels and the package requires the user to enter a pixel dimension (almost always a single pixel dimension, although OpenEXR is so old that it makes provision for non-square pixels).

Several camera systems provide enough metadata in their output to determine the dimensions and offset from optical sensor of this rectangle, or provide enough that when combined with static pixel dimensions from the manufacturer allow both dimensions and offset to be computed. Depending on the sophistication of the VFX artist or facility this can be easy or very tedious.

Defining a standard optional attribute to carry this rectangle's position and dimensions would encourage camera vendors and vendors of systems transcoding camera output to OpenEXR to properly populate it.

Note that this corresponds to the `ActiveSensorPhysicalDimensions` metadatum defined in [SMPTE's camdkit](https://github.com/SMPTE/ris-osvp-metadata-camdkit).

Start by reading the issue and the referenced SMPTE camdkit ActiveSensorPhysicalDimensions definition to clarify the rectangle's position and dimensions. The payload names no OpenEXR implementation files or tests; done means defining and integrating an optional attribute that can carry this metadata.

Written by the indexing model from the issue text.