Add swappable skybox texture

[mabelzhang]

Desired behavior

Sky box URI can now be changed via sdformat ( gazebosim/sdformat#1037 ), but that field is not being used by gz-rendering yet.

Currently, rendering uses a hardcoded stock sky texture in ogre2/src/media/materials/textures/skybox.dds.

Alternatives considered

Implementation suggestion

Change the code associated with this line:

gz-rendering/ogre2/src/Ogre2Scene.cc

Line 1478 in 2261309

std::string skyboxEnvMap = "skybox.dds";

which loads the stock texture skybox.dds at https://github.com/gazebosim/gz-rendering/blob/ign-rendering6/ogre2/src/media/materials/textures/skybox.dds

Instead, load a texture from the sky box URI tag.

Additional context

After rendering has been implemented, to add this to gz-sim, start in this line by checking children of <sky> SDF tag:

https://github.com/gazebosim/gz-sim/blob/f39bc25da4a93a0b2ccf5fca8c3e1b6863185a9d/src/gui/plugins/scene3d/Scene3D.cc#L3030