mesa: Set minimum possible GLSL version

Set the absolute minimum possible GLSL version.  API_OPENGL_CORE can
mean an OpenGL 3.0 forward-compatible context, so that implies a minimum
possible version of 1.30.  Otherwise, the minimum possible version 1.20.
Since Mesa unconditionally advertises GL_ARB_shading_language_100 and
GL_ARB_shader_objects, every driver has GLSL 1.20... even if they don't
advertise any extensions to enable any shader stages (e.g.,
GL_ARB_vertex_shader).

Converts about 2,500 piglit tests from crash to skip on NV18.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109524
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110955
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 0349bc3c)