opengl32: Automatically download the GL spec files in make_opengl. Add a default value for the OpenGL version.

Standardise on lower case "warning" and "error", since they match the
case of what gcc prints.

Previously, this was done partially (for fixed array types only).

startIdx should be the first index to draw, either from the vertex
array or the index array, depending on if the draw is indexed or
not. Having both at the same time wouldn't make sense.