Windows device support

[ChaseFlorell]

After having a conversation with @adamped on the Xamarin Forms forum [link], I think we need an open issue around Windows device support (WP8.1 SL/RT and UWP).
This is what Adam came up with as a starting point.

WP8.1 SL just auto scales downwards

  <add key="wp81" pathFormat="{0}\WP81\{1}" scale="1" />

WP8.1 RT

  <add key="wpa81-100" pathFormat="{0}\WPA81\{1}.scale-100" scale="2.4" />
  <add key="wpa81-140" pathFormat="{0}\WPA81\{1}.scale-140" scale="1.714285714285714" />
  <add key="wpa81-180" pathFormat="{0}\WPA81\{1}.scale-180" scale="1.3333334" />
  <add key="wpa81-240" pathFormat="{0}\WPA81\{1}.scale-240" scale="1" />

UWP

  <add key="uwp-100" pathFormat="{0}\UWP\{1}.scale-100" scale="4" />
  <add key="uwp-125" pathFormat="{0}\UWP\{1}.scale-125" scale="3.2" />
  <add key="uwp-150" pathFormat="{0}\UWP\{1}.scale-150" scale="2.66666667" />
  <add key="uwp-200" pathFormat="{0}\UWP\{1}.scale-200" scale="2" />
  <add key="uwp-400" pathFormat="{0}\UWP\{1}.scale-400" scale="1" />

[ChaseFlorell]

Note: We're still in the early stages, so we can always adjust the scale of the other platforms and require an even larger original image. Here's what I know now.

Given the expected image size on screen being 40 x 40, the input size needs to be 120 x 120 (currently).

Scale	Android	iOS	WP SL	WP RT	UWP
4	30 x 30 (ldpi)
3	40x40 (mdpi)	40x40 (1x)
2	60x60 (hdpi)
1.5	80x80 (xhdpi)	80x80 (2x)
1	120 x 120(xxhdpi)	120x120(3x)