Support alpha in Standard Surface to glTF PBR translation

[bowald]

closes #2579

This converts opacity to alpha as implemented in standard_surface.mtlx, i.e with the limitation of monochromatic opacity.

standard_surface.mtlx#L411-L424

 <!-- Surface construction with opacity --> <!-- Node <surface> only supports monochromatic opacity so use the luminance of input opacity color --> <luminance name="opacity_luminance" type="color3"> <input name="in" type="color3" interfacename="opacity" /> </luminance> <extract name="opacity_luminance_float" type="float"> <input name="in" type="color3" nodename="opacity_luminance" /> <input name="index" type="integer" value="0" /> </extract>

Opacity 1, 0, 0

Standard Surface	Translated to gltf_pbr, with alpha mode manual set to BLEND	Diff

[bowald]

Since alpha_mode is required to be set to BLEND in gltf_pbr to be render correctly if opacity is used. Should the translation graph also set alpha mode to BLEND if alpha not eq to 1?

Marking this PR as draft until we decide if the translation graph should handle alpha_mode as well.