This addresses an issue where OpenGL-backed image content could get lost
when putting and Android app into an inactive state. This happens
because the GL context gets destroyed, freeing all associated resources.
The workaround introduced here will listen for OpenGL state-change
events, and attempt to save and restore framebuffer content into main
memory, so that the content can be restored once the app is reactivated.
Since b108fe26c3, JUCE NSViews have
ignored mouse movements for child views. However, this causes issues for
OpenGL views, which still expect to receive mouse input via the parent
view. This fix makes the inner OpenGL view effectively invisible to the
mouse, so all mouse events are handled by the owning JUCE view.
