Support relative paths for PBR materials

[nkoenig]

Resolves #286.

Requires: gazebosim/sdformat#349.

To test, you can modify PBR map URIs similar to the following.

Old

<pbr>
  <metal>
    <albedo_map>https://fuel.ignitionrobotics.org/1.0/OpenRobotics/models/Cave Straight Type A/3/files/materials/textures/Gravel_Albedo.png</albedo_map>
    <normal_map>https://fuel.ignitionrobotics.org/1.0/OpenRobotics/models/Cave Straight Type A/3/files/materials/textures/Gravel_Normal.png</normal_map>
    <roughness_map>https://fuel.ignitionrobotics.org/1.0/OpenRobotics/models/Cave Straight Type A/3/files/materials/textures/Gravel_Roughness.png</roughness_map>
  </metal>
</pbr>

New

<pbr>
  <metal>
    <albedo_map>materials/textures/Gravel_Albedo.png</albedo_map>
    <normal_map>materials/textures/Gravel_Normal.png</normal_map>
    <roughness_map>materials/textures/Gravel_Roughness.png</roughness_map>
  </metal>
</pbr>

Signed-off-by: Nate Koenig [email protected]

[chapulina]

Works for me, I tried with this model: https://app.ignitionrobotics.org/chapulina/fuel/models/Rescue%20Randy%20-%20relative%20paths

[chapulina]

This is blocked by a libSDFormat8 release.

[chapulina]

@osrf-jenkins run tests, you can do it!

[chapulina]

@osrf-jenkins run tests, SDFormat 8.9.0 is out!

[codecov]

Codecov Report

Merging #328 into ign-gazebo2 will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo2     #328      +/-   ##
===============================================
+ Coverage        78.11%   78.13%   +0.01%     
===============================================
  Files              183      183              
  Lines            10029    10038       +9     
===============================================
+ Hits              7834     7843       +9     
  Misses            2195     2195              

Impacted Files	Coverage Δ
src/Conversions.cc	78.85% <100.00%> (+0.31%)	⬆️

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