When GL_POLYGON_OFFSET is enabled, each
fragment's depth value will be offset after it is interpolated
from the depth values of the appropriate vertices.
The value of the offset is "factor" ~*~ DZ ~~+~~ r ~*~ "units",
where DZ~ is a measurement of the change in depth relative to the screen
area of the polygon, and r is the smallest value that is guaranteed to
produce a resolvable offset for a given implementation.
The offset is added before the depth test is performed and before
the value is written into the depth buffer.
glPolygonOffset is useful for rendering hidden-line images, for applying decals
to surfaces, and for rendering solids with highlighted edges.