Fix window pos

[supersaiyansubtlety]

based on #307 because it adds Utils::clamp rebased

• fixes crash when Vec2i configs had missing components
• enures the main window is on-screen when opening (probably fixes broken configs can cause UI to silently fail #272)

---

I was testing #272 and I think the 2.1+ behavior described was because the window was off-screen:

2.1+: Creates an empty window that can't be interacted with (Can't maximize, bring to front, has no title, has no preview when hovering over the windows dock bar icon)

Though the window preview was blank, not missing or transparent:

The config attached to #272 has this:

# How big the Enigma window will open, in pixels.
# default: Vec2i[x=1024, y=576]
[window_size]
	x = 1936
	y = 1056

# The position the top-left corner of Enigma's window will be the next time it opens, in pixels.
# default: Vec2i[x=0, y=0]
[window_pos]
	x = -32000
	y = -32000

[supersaiyansubtlety]

ready!

[ix0rai]

looks good! i'm unable to reproduce the off-screen issue on KDE, but the resizing behaviour is much nicer with this PR