iOS: Clean up promotion of EAGLLayer use down to FBObject

Initial commit bba73bc096250a3c7fc036d84b1ea054d1b70b06 hacked
its path using a context global EGLLayer instance attachement.
The hack was good for the first demo, however, it forbid using
other FBObjects etc on the way.

Properly specifying FBObject.Attachment.StorageDefinition,
allowing the user to inject code for selected FBO attachements
to define their storage. This might be useful for different
platforms as well - however, it is OS agnostic and instance specific now.

In this sense, GLFBODrawableImpl, hosting FBObject,
has a more specific instance of FBObject.Attachment.StorageDefinition
for color-renderbuffer. It is passed along newly created color renderbuffer.

GLDrawableFactoryImpl.createGLDrawable uses a derived interface,
OnscreenFBOColorbufferStorageDefinition which is defined in
IOSEAGLDrawableFactory and return by its getter.
GLDrawableFactoryImpl.createGLDrawable is therefor platform agnostic again.

Bottom line is, as more platforms will be added, these semi-public interfaces
have to adapt to suit them all ..

All this due to iOS architecture for 'onscreen rendering' using a FBO
which shares its color renderbuffer storage with the EAGLLayer,
associated with the UIView. A bit weird maybe in first sight,
but efficient for creating cheap hardware design ;-)
Only criticism here is that Apple didn't bother using EGL and an extension.

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