Improve back-end interface, especially for GLES2 devices

Implement working support for EGL OpenGL-ES 2.0 framebuffer
console support for ARM-based Allwinner devices using Mali-400.
Performance is noticably slower than in X11, probably due to
limitation in the framebuffer Mali drivers.

The back-end interface has been refined in some places.

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