Different algorithm for imageCleanTransparent

[Desour]

• Goal of the PR: Make imageCleanTransparent faster.
• How does the PR work: First scale the image down, to all levels, up to 1x1. Then go the other direction, and bilinear sample the lower-resolution texture where the higher-resolution texture has alpha below threshold.
• See Texture modifiers and imageCleanTransparent are too slow #14322.

Results

Performance is noticeably better. But it's not optimal.

Left is master, right is PR:

To do

This PR is a Work in Progress.

• Make threshold always 0.
• Convert images to argb8, to remove old code.
• Clean up code.

How to test

• Test mod: https://codeberg.org/Desour/test_tmp/src/branch/imagecleantransparent_test (This branch, not master.)
• See Texture modifiers and imageCleanTransparent are too slow #14322 for performance testing.