Change minimum OpenGL version to 3.0 instead of 3.1

This broadens the range of cards supported.

Lowered the required OpenGL version from 3.1 to 3.0.
Lowered the required GLSL version from 1.40 to 1.30.

Very little actual code was changed, only the version checks.

Tested on Linux and Windows.
Tested on NVidia official, NVidia open source, AMD (unknown) and Intel integrated.