Fix text rendering op OpenGL-ES 2.0 platforms

Fix text rendering on OpenGL-ES 2.0 by working around limited precision
(as low as 14 bits) for integer uniforms.
Remove auto-generated file shaders_builtin.cpp
OpenGL-ES 2.0-related and other bug fixes, and update README

Fix single pass shader initialization when there are no lights.

Fix the attribute selection when EMISSION_ADD_DIFFUSE_REFLECTION_COLOR
is set for an object, and clarify the behaviour with the flag using
single pass shader 7.

Further changes to OpenGL-ES 2.0 text rendering, splitting the text
drawing into multiple requests when necessary. There are still
problems with the implementation, resulting text not showing up on
the screen.

Update README.

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