Correction to r54600

Calculate dependent area based on distortion model rather than
using 15% of image resolution for this.

Some assumptions here:

- We're assuming real-life camera calibration is used here
- Maximal undistortion delta would be achieved on frame boundary
- Distortion fully goes inside frame

This makes it possible to approximate margin for distortion by
checking undistortion delta across frame boundary and use it
for dependent area of interest.

We do not use any formula-based equation here because we're likely
support other distortion models and in that case it'll be stupid
to try detecting formula here.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54622 954f8c5b-7b00-dc11-b283-0030488c597c