[Monocle] Prism should use Monocle's platform detection code

On Embedded platforms we need to detect the platform for two features: EGL window/surface creation and hardware cursor.
The code for hardware cursor is in Glass (in Monocle in this case); the code for creating the EGL window and surface is in Prism/EGLFB.
We should allow an embedded back-end of Prism to talk directly to Monocle's NativeScreen, and make NativeScreen handle the platform-dependent parts of EGL setup.
This should be a new back-end for Prism: MonocleGL