Fix DPI scaling in examples

[dbr]

Extending from #504,

1. Using HiDpiMode::Default is indeed a good default
2. As noted in comments (and in comment on the linked PR), imgui-winit-support + winit handles most of the scaling for us
3. However imgui's default bitmap font then looks a bit naff (most likely why Rounded was used in first place), so I've changed the default font to Roboto as that was already in the repo
4. I've enabled oversampling on this font as it helps with the default rasterizer
5. The font rendering still then doesn't look too great due to imgui-glium-renderer's lack of proper compositing, so I've kept the "multiplier" value (but nudged down a little as I think it looks "better")

Overall I think this makes the DPI scale handling about as close to correct as it can be - so for example if I force the DPI scale to 1, and 6, it just looks like a 6x scale:

Point 5 does kind of hints we should either add some easy sRGB mode into glium-renderer, or switch the examples to using imgui-glow-renderer (which already does the correct thing by default)

[dbr]

Only thing I've not really tested is what happens when you drag a window between different DPI screens - winit emits an event for these, but I'm not sure if there is additional work needed

[sanbox-irl]

yeah i'd say this is totally fine. Nice work