gloo.gl: add dummy backend (each function raises error)

For when the rendering is done elsewhere.

Gloo initialize (for desktop/es2) compat implemented better
gloo.gl handle desktop/es2 compat in GLIR

Compat for initializing point sprites for desktop.
Compat for modifying shading code.
Program can now read variables with precision qualifiers.
Qt now supports EGL on Linux. We can run real ES 2.0!