main: Fix deprecation of glLineWidth()

From section E.1 (Profiles and Deprecated Features of OpenGL 3.0)
of the OpenGL 3.0 spec:

    "LineWidth is not deprecated, but values greater than 1.0
    will generate an INVALID VALUE error"

From context it is clear that values greater than 1.0 should only
generate an INVALID VALUE error in a forward-compatible context.

The code was correctly quoting this spec text, but it was disallowing
all line widths in forward-compatible contexts, instead of just widths
greater than 1.0.

This patch introduces the correct check, so that setting a line width
of 1.0 or less is permitted.