fix: Contain not preserving aspect ratio of tall images

When both width and height is set to `Contain`, the scaling uses the
wrong direct proportional scaling formula for tall images. This is due
to `aspect` being `width / height`:
By dividing (original), the formula expands to `scaled_width /
scaled_height : height / width` which inversely scaled the width of tall
images. By multiplying instead, the formula correctly expands to
`scaled_width / scaled_height : width / height` which preserves aspect
ratio when scaling.

wezterm-gui/src/termwindow/background.rs

@@ -471,7 +471,7 @@ impl crate::TermWindow {
             }
         } else {
             // Height is the longest side
-            let target_width = pixel_height / aspect;
+            let target_width = pixel_height * aspect;
             if target_width > pixel_width {
                 (
                     pixel_width,