gsk: Use the appropriate GLSL version

gsk: Use the appropriate GLSL version

The GLSL versions are:

  OpenGL 2.1: #version 110
  OpenGL 3.0: #version 130
  OpenGL 3.2: #version 150

  OpenGLES 2.0: #version 100
  OpenGLES 3.0: #version 300 es

So we need to check the version of the GdkGLContext if we want use the
appropriate version, especially for legacy OpenGL contexts, which can be
both 3.x and 2.x.