Particle example doesn't compile with USE_OPENGL_CONTEXT off

[SoylentGraham]

Working on windows, so seeing testing without the opengl context...
Will work on a pull-request when I get a chance, but for now; (Just to get it compiling)

• change vec2 to float2
opencl.readBuffer(... should be

clMemPosVBO.read( particlesPos, 0, sizeof(float2) * NUM_PARTICLES, true );

and I've just commented out OpenCL::setupFromOpenGL entirely as variables are in the wrong place