v3.0.8-v1.4.0: fix: remove unnecessary screen width rounding in ChangeScreenSize (#11)

## Summary
- Removes the Go-level `s.Width = s.Width - (s.Width % 8)` rounding in
`ChangeScreenSize` that was silently truncating requested widths (e.g.
390 → 384)
- Makes `XCreateScreenMode` return the actual dimensions that `libxcvt`
produced via in/out pointer parameters, so the Go caller uses the
correct width for `XSetScreenConfiguration`
- Fixes the mode name in `XCreateScreenModeInfo` to use `libxcvt`'s
actual output dimensions instead of the original request, preventing
name/geometry mismatches

## Context

The rounding was originally added to match `libxcvt`'s CVT convention of
producing widths as multiples of 8. However, CVT is a timing standard
for generating timing parameters for physical monitors — not a hard
requirement of X.Org or the Xorg dummy driver used for virtual displays.
The rounding made it impossible to use non-multiple-of-8 resolutions
like 390x844 for mobile viewports, even when valid modelines existed in
`xorg.conf`, because Neko would turn 390 into 384 before ever searching
for a mode.

Now the resolution flow is:
1. **Exact match** — if a pre-defined modeline exists in `xorg.conf` for
the exact requested dimensions (e.g. `390x844`),
`XSetScreenConfiguration` finds and uses it directly.
2. **Dynamic creation** — if no match, `XCreateScreenMode` dynamically
creates a new mode via `libxcvt` and reports the actual dimensions back
to the caller.

## Verified

Tested end-to-end in a headful `chromium-headful` container:
- Requested 390x844@25Hz via the kernel-images API
- xrandr confirmed display at exactly `390x844_25` (using a manually
crafted modeline in `xorg.conf` that bypasses CVT rounding)
- CDP verification confirmed `screen.width=390`, `screen.height=844`,
`innerWidth=390`, `innerHeight=807` (37px app-mode title bar)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches XRandR mode selection/creation and changes the
resolution-setting flow, which could affect display configuration
behavior across environments, but the change is localized to the Xorg
wrapper.
> 
> **Overview**
> `ChangeScreenSize` no longer truncates widths to a multiple of 8
before attempting to apply a screen mode; it now tries to set the exact
requested dimensions first and only creates a new mode if no existing
mode matches.
> 
> `XCreateScreenMode` is updated to take width/height as in/out pointers
so the caller receives libxcvt’s *actual* generated dimensions, and
`XCreateScreenModeInfo` now names modes using those actual dimensions to
avoid name/geometry mismatches.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3f90ca61f777dcfca35fa7d3538e365c3ad84cd5. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

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Co-authored-by: Cursor <cursoragent@cursor.com>