Improve implementation of GL_POINT_SPRITE_COORD_ORIGIN errors

Improve implementation of GL_POINT_SPRITE_COORD_ORIGIN errors

This enum is only supported for OpenGL 2.0.  If a driver supports
OpenGL 1.4 and GL_ARB_point_sprite, using this enum should generate an
error.  This is important because, for example, i915 and i830 can
support GL_ARB_point_sprite, but they cannot support
GL_POINT_SPRITE_COORD_ORIGIN.

This commit just removes the check for NV_point_sprite, which is
completely wrong, and add some comments describing what the code
should do.  I don't see an easy way to check for version >= 2.0 from
inside Mesa.  Perhaps we should add an extension
GL_MESA_point_sprite_20 (like Intel's old GL_EXT_packed_pixels_12) to
indicate that this added bit of functionality is available.

Also note that glean's pointSprite test only checks for
GL_ARB_point_sprite before trying to use
GL_POINT_SPRITE_COORD_ORIGIN.  Naturally, that fails on
non-2.0 implementations (i.e., Mac OS X on GMA 950).