- For point picking, the glyph mapper sets the point size using
fixed function method. In order to reliably select points with
reasonable tolerances, VTK always sets a point size of greater than 1 during
the point selection pass. This is how point picking has
worked in all the VTK mappers.

- The problem was that the `glPointSize(pointSize)` function is unreliable
in few OpenGL implementations, particularly Angle's implementation of OpenGL over
D3D11. This implementation is used in Chrome/Edge on windows. As a
result, on windows, the function `glPointSize` never did anything and
pointSize was always 1.0 which lead the point picker to consistently
fail.

- The solution in this commit sets a variable `gl_PointSize` from the vertex
shader which is supported by webgl2 and desktop implementations as well.

- For now, the mapper still uses the fixed function `glPointSize` on desktop and only
sets `gl_PointSize` from the vertex shader when GLES 3.0 is being
used.