Support DPI

[ryantrem]

Babylon.js has some support for scaling the display in different ways, based on the adaptToDeviceRatio Engine constructor parameter, Engine.setHardwardScalingLevel, and window.devicePixelRatio. I believe Babylon Native does not currently support any of this, and that currently we always specify view size in device pixels. I think that also means on mobile devices we are probably rendering at very high resolutions, perhaps higher than necessary with an associated resource cost. We should think about how to support this as it could have a huge impact on mobile performance.

[Drigax]

I think this is something worth investing in, especially for the potential performance gains as we would potentially be able to upscale from lower rendered resolutions. I'll start looking into this

[Drigax]

I added some rudimentary support for NativeEngine.setHardwareScalingLevel via this branch. One thing of note is that in BabylonJS hardware scaling is a reciprocal function, so that behavior needed to be preserved.: https://github.com/Drigax/BabylonNative/tree/add-engine-hardware-scaling-level

This is also dependent on a change to BabylonJS which properly calls these new NativeEngine members.

I'll open a PR Thursday when I return and begin looking into implementing a polyfill for window.devicePixelRatio

[Drigax]

Looking at MSDN:
https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio

The devicePixelRatio of Window interface returns the ratio of the resolution in physical pixels to the resolution in CSS pixels for the current display device. This value could also be interpreted as the ratio of pixel sizes: the size of one CSS pixel to the size of one physical pixel. In simpler terms, this tells the browser how many of the screen's actual pixels should be used to draw a single CSS pixel.

This gives me the impression that window.devicePixelRatio can be implemented native-side as another scalar applied to calculate the resolution of the BGFX window. I'll most likely implment this similarily to how I'm currently implementing Engine.hardwareScalingLevel

[ryantrem]

I think we can deduce devicePixelRatio on iOS/Android with:
iOS: UIScreen.mainScreen.nativeScale
Android: DisplayMetrics.density
I'm not 100% sure this is right, so we'll need to test it out.

[Drigax]

#582