[Libretro] Fix VMU scaling for dynamic res games

[bslenul]

Noticed while testing #1339, the on-screen VMU can get squished if the game decides to change its resolution.

Before:

After:

[bslenul]

Erm, VMU is getting stretched now when widescreen hack is enabled... 🤔

edit: Added

if (config::Widescreen)
{
	vmu_padding_x = vmu_padding_x / 4.f * 3.f;
	vmu_width = vmu_width / 4.f * 3.f;
}

for VK and D3D11, somehow GL handles this perfectly fine already.