Update RenderingPlugin.cpp: postponed initialization to OnRenderEvent() to avoid invalid GL Context handle

[AnimatorJeroen]

With this minor change, the example project becomes compatible with using mcpiroman's dynamic dll loading unity package.

The problem was that there is no handle to the active OpenGL context during OnPluginLoad, when using the dynamic loading package.

With this minor adjustment, the renderer initialization is postponed to the first OnRenderEvent(), where there is a valid GL Context.

This way the openGL profile will run in combination with mcpiroman's plugin, making it possible to change the dll without restarting Unity.