gl_VertexID — contains the index of the current vertex
in highp int gl_VertexID ;
gl_VertexID is a vertex language input variable that holds an integer index for the vertex. The index is impliclty generated by
glDrawArrays and other commands that do not reference the content of the
GL_ELEMENT_ARRAY_BUFFER, or explicitly generated from the content of the
GL_ELEMENT_ARRAY_BUFFER by commands such as
|OpenGL ES Shading Language Version|
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