Make compatible with OpenGL3+, make legacy code optional

[Jeremy Hill]

Shady currently works best on (Windows) systems that allow legacy and modern features to be mixed. In legacy-only OpenGL (e.g. the default context on macOS) it also works but random-number generation is poorer. We need to create a modern-OpenGL-only mode, both for improved functionality on macOS and for forward compatibility:

• update shader code (done)
• remove reliance on fixed-function pipeline and matrix stacks, i.e. glMatrixMode..... glTranslate.... glRotate...., (done)
• provide VBO/VAO alternative to immediate-mode drawing commands glBegin... glVertex... glEnd, (partially done: moderngl branch only, PyEngine only)

Tests:

• Python 2, Python 3
• Test unaccelerated backend with pyglet.GL as well as OpenGL.GL
• bare (without numpy and friends)

[Jeremy Hill]

Also drop pyglet support unless/until pyglet can create 3.3 contexts (see scraps/pyglet_opengl3_attempt.py). Note that there is a Python package that binds to GLFW.

[Jeremy Hill]

Done and looks good. New --legacy option interacts with --openglContextVersion allows implementations to be compared. Until extensive tests are done legacy=True remains the default option.