gl_Position — contains the position of the current vertex
out highp vec4 gl_Position ;
The variable gl_Position is intended for writing the homogeneous
vertex position. It can be written at any time during vertexshader execution. This value
will be used by primitive assembly, clipping, culling, and other fixed functionality
operations, if present, that operate on primitives after vertex processing has occurred.
Its value is undefined after the vertex processing stage if the vertex shader executable
does not write gl_Position.