Fix excessive memory usage when saving EXR file with many channels

[brechtvl]

Description

Scratch memory allocation in ImageOutput::to_native_rectangle was accidentally using the number of channels squared.

We encountered a case where saving a 2K EXR file with 107 channels would use more than 50GB.

I don't have a simple isolated repro case. It requires writing tiles with a stride that does not match the stride in the image file. But hopefully the error in the code is clear.

Tests

I did not add a test, it's not clear there is a good mechanism to verify memory usage like this with a test.

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

Holy crap! That's an embarrassing one. Good catch!

I will merge as soon as the CI finishes.