smyrna: avoid inadvertent double promotion in 'renderSelectedNodes'

smyrna: avoid inadvertent double promotion in 'renderSelectedNodes'

Squashes two compiler warnings:

  topviewfuncs.c: In function ‘renderSelectedNodes’:
  topviewfuncs.c:283:41: warning: conversion from ‘double’ to ‘GLfloat’ {aka
    ‘float’} may change value [-Wfloat-conversion]
    283 |             glVertex3f(pos.x,pos.y,pos.z+0.001);
        |                                    ~~~~~^~~~~~
  topviewfuncs.c:285:50: warning: conversion from ‘double’ to ‘float’ may
    change value [-Wfloat-conversion]
    285 |             drawCircle(pos.x,pos.y,nodeSize,pos.z+0.001);
        |                                             ~~~~~^~~~~~