Fix and optimize RPi back-end; text and other shader optimizations

The RPi back-end now works well, with fairly good performance for
simple scenes. The text shader has been extended to work around
limitations in OpenGL-ES 2.0 and the RPI's implementation (it now
supports floating point character values, and draws up to eight
characters at a time).

The precision used for some values in fragment shaders has been
reduced (without much degradation in quality) for OpenGL-ES 2.0
platforms for improved performance.

The RPi's VideoCore provides higher quality and better performance
than standard Mali-400 with available drivers, although stencil
shadow volumes (which are slow anyway on OpenGL-ES 2.0 devices)
suffer from pixel artifacts in more complex scenes, suggesting
that the RPi's rendering is not deterministic.

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