gl2ps removal from OCCT dependencies

Historically, OCCT 3D Viewer provided two options for dumping view content:

• V3d_View::ToPixMap() saving an image screenshot (bitmap) into formats like PNG, BMP, JPG.
• V3d_View::Export() exporting into vector graphics image format into formats like SVG, PDF, PS.

The latter one was provided with help of an external library gl2ps basing on OpenGL transformations feedback functionality (when OpenGL commands issued for rendering are captured and stored into dedicated buffer).

There were many issues with this functionality - different results depending on OpenGL driver vendor, driver bugs, missing elements in result, high memory consumption and very heavy output file for complex scenes. However, the key reason for deprecation of V3d_View::Export() in OCCT 7.1.0 was incompatibility of underlying OpenGL functionality (deprecated since OpenGL 3.1 and undefined within OpenGL ES) with modern rendering techniques making vector image dump just impossible in usual way (e.g. just by using the same rendering code path).

gl2ps support will not be enabled within forthcoming official release OCCT 7.3.0, and next release will completely remove this functionality. The users of removed functionality (if any) should consider using HLRBrep package and export 2D views using general Data Exchange tools based on BRep definition.