Improvement of text output in viewers on various supported platforms

This improvement includes several tasks:

1. Implementing texture-mapped fonts (TMF) on Linux (and on other supported
platforms, if it is efficient and feasible), on the basis of OCC2934 results.

2. Improving TMF on Windows (and on other platforms, in case if this
improvement is not a feature of TMF implementation for some platfrom): it is
necessary to provide a possibility to change font size for TMF output. The
easiest way is to use the same texture image for all font sizes and to scale
fonts using OpenGL interpolation - quality of resulting scene to be checked!

3. Improving Graphic3d_AspectText3d interface: provide a way to specify the
font to be used for text output directly by font name string (instead of
enumeration) on all supported platforms (primarily on Windows and Linux,
optionally on the others if feasible).

As the first step, developer's specification should be created for each of
these tasks and atached to this Bugtracker record. Specifications should
describe proposed modifications to OpenGl package (new functions, modifications
to existing ones, changes of data structures) and higher-level packages
(Graphic3d, InterfaceGraphic). Connection from all high-level parameters down
to OpenGl data structures and function calls should be also specified clearly.