Remove notion of default rendertarget. This doesn't map well to usage patterns outsid...

Remove notion of default rendertarget. This doesn't map well to usage patterns outside sample app. Make binding between SkGpuDevice and a GrRenderTarget more explicit. Create method on GrContext to wrap the current target in the 3D API with a GrRenderTarget.

git-svn-id: http://skia.googlecode.com/svn/trunk@706 2bbb7eff-a529-9590-31e7-b0007b416f81