Never increase dimensions when applying alignment

Previously, the minor dimension was rounded to the nearest multiple of
the alignment requirement when constraining size. This could result in
the dimension being increased.

Change the behavior to always round down instead, ensuring the
constrained size never grows.

This fixes a conflict with the client-side "optimal window size"
computation, which never increases dimensions. With flex displays, the
old behavior could lead to feedback loop between window and display
resizing with mismatched dimensions:

 - window  resized to 2341x1317
 - display resized to 2340x1318
 - window  resized to 2338x1317
 - display resized to 2336x1318

PR #6772 <https://github.com/Genymobile/scrcpy/pull/6772>