Add support for OpenGL ES 2.0 shader

[Risca]

I was hoping I would be able to use this library on an old BeagleBoard that I had lying around (or a raspberry pi), but the BeagleBoard only support OpenGL ES 2.0 and I was unable to compile the LineNode.vsh vertex shader:

QOpenGLShader::compile(Vertex): Compile failed.
ERROR: 0:1: Syntax error, GLSL Version 120 not supported
ERROR: 1 compilation errors. No code generated.

Unfortunately, I suck at shaders so I open this issue in the hopes that someone more knowledgeable could help me out. I'm happy to help out with testing.