Dissociate virtual display size and capture size

Allow capturing virtual displays at a lower resolution using
-m/--max-size.

In the original implementation in #5370, the virtual display size was
necessarily the same as the capture size. The --max-size value was only
allowed to determine the virtual display size when no explicit size was
provided.

Since the dpi was scaled down accordingly, it is often better to create
a virtual display at the target capture size directly. However, not
everything is rendered according to the virtual display DPI. For
example, a page in Firefox is rendered too big on small virtual
displays. Thus, it makes sense to be able create a virtual display at a
given size, and capture it at a lower resolution with --max-size. This
is now possible using OpenGL filters.

Therefore, change the behavior of --max-size for virtual displays:
 - it does not impact --new-display without size argument anymore (the
   virtual display size is the main display size);
 - it is used to limit the capture size (whether an explicit size is
   provided or not).

This new behavior is consistent with main display capture.

Refs #5370 comment <https://github.com/Genymobile/scrcpy/pull/5370#issuecomment-2438944401>
Refs <https://github.com/Genymobile/scrcpy/pull/5370>

doc/virtual_display.md

@@ -8,7 +8,6 @@ To mirror a new virtual display instead of the device screen:
 scrcpy --new-display=1920x1080
 scrcpy --new-display=1920x1080/420  # force 420 dpi
 scrcpy --new-display         # use the main display size and density
-scrcpy --new-display -m1920  # ... scaled to fit a max size of 1920
 scrcpy --new-display=/240    # use the main display size and 240 dpi
 ```