Basic HDRP export support

[krisrok]

This adds basic export support for some of HDRP's standard shaders:

• Normal map
• Metallic/smoothness via scalars or mask map
• Occlusion via mask map
• Emission via HDR and LDR+intensity workflows and emission map

To support the remapping of the mask map values, the channel swap shaders now support min/max values per used channel.
smoothnessMultiplier is now smoothnessRangeMax and smoothnessRangeMin has a default value of 0. Newly introduced min/max ranges also default to 0/1. This way, it should work exactly like before for material exports not using the ranges.

There's much to be desired for full support, even in the added functionality: UV sets, multiplicative emission, emission exposure weight come to mind. But for my use cases it works just fine and produces good looking results.

[CLAassistant]

All committers have signed the CLA.

[hybridherbst]

Thanks for the PR! We'll take a look. Are you planning on adding the remaining changes that are needed to integrate it, e.g. into the animation system? (e.g. different treatment for emissive color animation and so on)