Intel's Mesa Driver Tacks On Another GL Extension For Old Hardware
If you're stuck with an older Intel Gen 4 or Gen 5 graphics processor, with this commit the GL_EXT_polygon_offset_clamp extension is supported. The patch for this enablement on the older Intel IGPs was done by prolific community contributor Ilia Mirkin as opposed to being from the Intel OTC staff.
As described at OpenGL.org, "This extension adds a new parameter to the polygon offset function that clamps the calculated offset to a minimum or maximum value. The clamping functionality is useful when polygons are nearly parallel to the view direction because their high slopes can result in arbitrarily large polygon offsets. In the particular case of shadow mapping, the lack of clamping can produce the appearance of unwanted holes when the shadow casting polygons are offset beyond the shadow receiving polygons, and this problem can be alleviated by enforcing a maximum offset value."