class ZGlProgram: public ZList

 Representation of a GLSL program.
 Children are enforced to be ZGLShader instances.

 ZGlProgram_GL_Rnr compiles all the shaders and links the program.

 Unifrom variables are extracted and information is stored in the local
 map. Defaults are parsed out of the shader source and stored as string.
   uniform vec3  BrickColor;  // (1.0, 0.3, 0.2)
 If bSetUniDefaults is set (default), these values are assigned to
 appropriate GL uniform variables when the program is activated.
 See also SGlUniform and SGlUniform_GL_Rnr.


 TODO:
 1. GL_Rnr::Draw() does nothing - can only be via state-modifier link.
 2. Add class for holding alternate uniform variable values.
    One could have a ZList (or ZNameMap) and use name, title of elements
    for variable name, value string - but that is burning memory.
    Better to have glass ZGlUniformValues that holds the lot in a list
    and marks in the GL_Rnr which are really available.
    ZGlProgram would have to become ZGlRnrMod for that, to allow for
    optimized detection of program state via time-stamps.
    Special Fltk class would have to be written.
    In any case, the univorm-values could be populated from a succesfully
    compiled program - all information is already available in mUniMap.
 3. Why is ZImage not a RnrMod? Should we separate ZTexture and ZImage?
    This might become relevant for sampler uniform varaibles.