Fixes compilation, and adds ability to capture animations of overlapping views, OpenGL support too.

[MattFoley]

Example use case:

Using this technique, I was able to capture images for two auxiliary views which while not animating themselves, were overlapped by an animations from another view and so would need to animate on the Watch to keep the animations seamless.

I did this by containing all target views for rendering in a clear superview, and ran multiple flipbooks simultaneously. In my case I was transitioning images, so I used UIViewContentModeCenter, with clip subviews disabled.

Example: You need to animate view A, which overlaps view B and C. You can now create three flipbooks simultaneously by passing in "frameInSuperview:true" and then on the watch play all three produced animations simultaneously for a seamless animation.