Implement built-in widescreen

[DCxDemo]

Implement proper built-in widescreen, provide options for custom aspect ratios. QScript menus are hardcoded for something like 640x448 ps2 screenbuffer, gotta find proper solution for GUI.

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The question is, should we recommend ThirteenAG's fix for widescreen support? Recent repo owner reply implies that known bugs are not the priority and unlikely to be fixed. https://github.com/ThirteenAG/WidescreenFixesPack/issues/392

Known issues with widescreen fix:

• buttons are still physically stretched 4:3
• UI is shrinked in the middle of the screen instead of being attached to the edges (due to the stretching the original virtual 640x448 screen instead of a more elaborate fix).
• some UI element quads have their vertex colors altered.

[Vadru93]

Well I think we can still recommend it, however implementing our own is already on my todo. But guess it's not prio 1.