Fix stories 9:16 export

[bjtitus]

Fixes tumblr/kanvas-ios#62

Testing

• Create a Story post
• Add landscape and portrait media
• Add text to the media
• Ensure that the media posted uploaded to the site is roughly 9:16. (about 0.5625 when dividing the width by the height)
• Ensure text is positioned as expected

Regression Notes

1. Potential unintended areas of impact

Anything related to Story media export.

2. What I did to test those areas of impact (or what existing automated tests I relied on)

Tested posting stories with a variety of media sizes and types.

3. What automated tests I added (or what prevented me from doing so)

Unfortunately, the OpenGL rendering will not work in the simulator so we cannot test this.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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