Gephi visualization engine

This is a new visualization engine for Gephi based on modern OpenGL techniques.

It aims to be:

• Retro-compatible with old OpenGL versions through feature discovery, falling back to the best supported by the graphics card
• High performance using most modern OpenGL when available, specially due to instancing, manual buffer management, using simple shaders and avoiding memory allocation when possible
• Extensible with plugins (rendering and input)
• LWJGL3 version (GLFW/AWT)
• Nicely interactive with mouse, directional zooming, etc with default input handler
• Only a 2D engine for the moment
• The only gephi-related dependency is graphstore

Currently, in comparison to Gephi 0.9.2 renderer it's lacking:

• Self loops
• Node/edge text labels