x/mobile/app: detect device orientation

[rakyll]

A function that returns the device's current orientation would be handy to switch between portrait- and landscape-optimized user interfaces.

const (
   OrientationLandscape = iota
   OrientationPortrait
   // ...
)

func Orientation() int

Thoughts? cc/ @crawshaw @hyangah

[hyangah]

Agree it would be nice.
But I'm not sure whether it belongs to mobile/app (what's the boundary of 'app' package?)
or we need a separate package (something corresponds to UIDevice or WindowManager).

[rakyll]

I would prefer to put it in window package. The device word restricts the functionality to the mobile devices.

[bonly]

it is important to get the screen's size when developing with mobile game! plz

[gopherbot]

CL https://golang.org/cl/9708 mentions this issue.

[rakyll]

This is also critical when working with sensor data, e.g. screen rotates but the accelerometer sensor is in the same position. The values read should be interpreted differently depending on the orientation.