Fix size of light visual

[peci1]

Light visual is displayed wrong - it shows a cone corresponding to a light with double the angle than it should represent.

Example light:

<sdf version='1.6'>
  <world name='default'>
    <model name='lights'>
      <link name='link'>
        <light name='light_up' type='spot'>
          <attenuation>
            <range>60</range>
            <linear>0</linear>
            <constant>0.1</constant>
            <quadratic>0.0025</quadratic>
          </attenuation>
          <diffuse>0.8 0.8 0.5 1</diffuse>
          <specular>0.8 0.8 0.5 1</specular>
          <spot>
            <inner_angle>0.7</inner_angle> <!-- about 40 degrees -->
            <outer_angle>0.7</outer_angle>
            <falloff>1</falloff>
          </spot>
          <direction>0 0 -1</direction>
        </light>
        <kinematic>1</kinematic>
      </link>
    </model>
    <model name='unit_box'>
      <pose frame=''>0 0 -0.7 0 -0 0</pose>
      <link name='link'>
        <visual name='visual'>
          <geometry>
            <box>
              <size>1 1 1</size>
            </box>
          </geometry>
          <material>
            <script>
              <name>Gazebo/Grey</name>
              <uri>file://media/materials/scripts/gazebo.material</uri>
            </script>
          </material>
        </visual>
        <kinematic>1</kinematic>
      </link>
    </model>
  </world>
</sdf>

It seems the math is just wrong in https://github.com/osrf/gazebo/blob/db40ad5e02f21e68e9924d725a49b3e107fecb45/gazebo/rendering/Light.cc#L314-L327

If we denote inner_angle by α, then the computation done in code to determine the corners of the pyramid is:

(±r*tan(α), ±r*tan(α), r)

Looking from a side, the situation is drawn here:

It follows from the top half-triangle that tan(α) = (r*tan(α))/r = (angles[i]/r) and thus α = atan(angles[i]/r)... But as angles[i] is just a multiple of tan(α), it is apparent that the angle at the apex of the upper half-triangle is inner_angle. Then sum it up with the lower half-triangle, and you get angle at the apex of the light visual 2*inner_angle.

If I add a light with halved inner_angle, the borders of the visual match the circle of the light exactly:

This PR just applies the halving of the angle to fix the visuals.

[peci1]

I also noticed the visual doesn't follow the direction specified in <direction> SDF tag. But I don't have an easy fix for that (although I'm sure it's just about constructing a rotation matrix and applying it to all the vertices; I don't have a Gazebo source build at hand to try that out).

[peci1]

What's interesting is that if I add the light via GUI, until I click to finalize its placement, its size is correct. As soon as it gets attached to the world, the field of view doubles.

[peci1]

Now it seems that the temporary light used when adding light interactively in GUI has wrong angle: #2981.