Fix and improve rendering of logo.avi banners in WON-style menus.

[Razcoina]

Fixes missing logo.avi banner in WON style menus.

Requires xash3d-fwgs PR #2451
Fixes xash3d-fwgs issue #2424

Changes include:

• Proper precaching and rendering of logo.avi
• Correct banner positioning across resolutions and aspect ratios
• Allow loading logo.avi from a fallback source (typically valve/media/logo.avi)
• Optional background stretching via a new cvar (defaults to existing behavior)

The banner is now scaled and positioned relative to the original 640x480 WON layout, ensuring consistent alignment regardless of screen resolution.

New CVAR
ui_background_stretch [0|1] - default: 0

Behavior:

• When 0: Crop menu background image and preserve aspect ratio
• When 1: Stretch menu background image to fit the current resolution

At 1920x1080:
ui_background_stretch 0:

ui_background_stretch 1:

[a1batross]

Looks incredibly good for your first PR here.

Thanks, good job!