Support <anti_aliasing> element when setting up camera render texture

[WilliamLewww]

Depends on:

• Anti-aliasing element for <camera><image> sdformat#909

The level of MSAA is hard-coded to 4. This pull request allows users to change the MSAA level with the SDF element <anti_aliasing>.

<camera name="camera">
  <image>
    <width>640</width>
    <height>480</height>
    <format>R8G8B8</format>
    <anti_aliasing>4</anti_aliasing>
  </image>
...
</camera>

[scpeters]

we'll probably need to check which version of libsdformat9 we find to ensure it is new enough

[scpeters]

we'll probably need to check which version of libsdformat9 we find to ensure it is new enough

and we'll need a new libsdformat9 release

• gazebosim/sdformat@sdformat9_9.7.0...sdf9

[WilliamLewww]

Waiting for the following pull request to make a sdformat9 release:

• Added <shininess> to <material> sdformat#985