Ideas for further improving performance of partially decoded previews

[hrj]

• [X] When partially decoding, do transformations before interpolation. It seems to kind of work out-of-the-box, but there glitches which are probably overflows into neighbouring pixels.
• [ ] An option to do simple interpolation. Instead of interpolating one zoom level at a time, find dx for top & bottom rows and dy for left & right rows. Then find the second derivative dx2 and dy2 and keep incrementing and filling pixel values line by line.
• [ ] An option to skip scaling by the library. Instead, the caller could use their own scaling method, which might be faster. For example, native canvas scaling in browser.

[mgifford]

Performance is an issue raised by the Mozilla community https://bugzilla.mozilla.org/show_bug.cgi?id=1240692#c6