Fix aspect ratio accuracy

The window aspect ratio was previously enforced by calling
`SDL_SetWindowAspectRatio()` before explicitly setting the window size,
using the width/height ratio of the new size.

However, this size may already be a scaled version of the actual content
size, subject to integer rounding. Using this derived size instead of
the actual content size could result in an inaccurate ratio, causing a
mismatch between the content and window aspect ratios, which may result
in a thin row or column of black pixels.

To avoid this issue, always set the aspect ratio based on the content
size.

Refs 1b4fd672869dcaa7f0ba8f11b722b731488d690d
Refs #6761 <https://github.com/Genymobile/scrcpy/pull/6761>
PR #6774 <https://github.com/Genymobile/scrcpy/pull/6774>