gl_PointCoord — contains the coordinate of a fragment within a point
in mediump vec2 gl_PointCoord ;
gl_PointCoord is a fragment language input variable that contains
the two-dimensional coordinates indicating where within a point primitive the current
fragment is located. If the current primitive is not a point, then values read
from gl_PointCoord are undefined.
gl_PointCoord.s ranges from 0.0 to 1.0 across the point horizontally
from left to right. gl_PointCoord.t varies
from 0.0 to 1.0 vertically from top to bottom.