Update OpenGL-ES2 back-end for RPi

The back-end for the Raspberry Pi now compiles, but there are
problems due to limitations in the VideoCore's shader language
support.

The back-end interface has been tweaked a little. For EGL back-ends
the native window is now created before the EGL initialization is
started.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>