x/mobile/app: faster reshape callback on darwin_amd64

[bronze1man]

I write a simple 2d app with gomobile on mac.
I paint one image with fixed position and fixed size.
I found that when I resize the window quickly ,the image size on the screen is bigger or smaller proportionally to the screen size.
Sometimes,the image size remember the windows size in seconds ago.

After I comment out following code,the problem disappear:

src/golang.org/x/mobile/app/darwin_amd64.m:48

It looks like the reshape function is too slow.
May be we can cache same result from the system api.

gomobile version +0a85ca5 Fri Dec 25 00:41:33 2015 +0000 (android,ios); androidSDK=/usr/local/AndroidSdk/platforms/android-22
go version go1.5.2 darwin/amd64

[hyangah]

/cc @crawshaw @nigeltao