AssimpImporter: support PBR

[mosra]

Postponed from mosra/magnum#459, custom material attribute support offloaded to #116.

List of nightmares to wake up from, most of them hopefully obsoleted with the allegedly better PBR support in 5.1:

• [ ] COLLADA ambient texture gets imported as LIGHTMAP (but ambient color is ambient color), however glTF occlusion texture gets imported as LIGHTMAP as well, what the heck do I do here? Why isn't glTF occlusion texture imported as AMBIENT_OCCLUSION if there's such a type ffs?!
• [x] import glTF PBR properties
• [ ] import other PBR properties (like, there's DIFFUSE_ROUGHNESS etc, what to do with that? which format uses it?)
  • it's a terrible mess, with each file format defining its own set of properties, properties duplicated, undocumented, ugh (https://github.com/assimp/assimp/issues/3949, https://github.com/assimp/assimp/pull/3858, https://github.com/assimp/assimp/issues/2740...)
• [ ] OBJ PBR https://github.com/assimp/assimp/issues/2944
• [ ] glTF unlit detection, as shown in https://github.com/assimp/assimp/issues/3491
• [ ] FBX PBR support since https://github.com/assimp/assimp/pull/3519
  • but wait, there's Maya PBR, 3dsMax PBR, Stingray PBR, ...?! https://github.com/assimp/assimp/issues/2740, https://github.com/assimp/assimp/pull/3858
• [ ] glTF BaseColor gets imported as Diffuse, detect that (using ~~other attribs?~~ importer name as in #97) and fix
• [ ] glTF metallic/roughness gets imported as UNKNOWN, detect that (using ~~other attribs?~~ importer name as in #97) and fix
• [ ] glTF *TextureCoordinates get imported as something else entirely for no reason, why?!
• [ ] glTF specular/glossiness texture gets imported as just Specular, with glossiness ignored, detect that and fix
• [ ] glTF files have one extra layer with the diffuse texture duplicated, why the fuck?! mention that in the docs at least
• [ ] what's that $mat.gltf.pbrSpecularGlossiness thing?
• [ ] detect presence of glTF metallic/roughness and specular/glossiness attributes and set MaterialType accordingly (or are those present always for any glTF files? ffs)
• [ ] disable the PBR enums and codepaths on older versions of Assimp somehow so we can still compile on Travis
• [ ] test the new code paths
  • [ ] glTF import will probably crash on all except the very latest versions, skip those there
• [x] importing unknown attributes as custom -- done with #116
  • [x] an ability to import all, not just unknown, attributes as "raw" without trying to make sense of those
  • [x] test those code paths

Current output from magnum-sceneconverter for an overview of how bad things are:

$ magnum-sceneconverter --info WaterBottle.gltf 
Material 0 (referenced by 1 objects): BottleMat
  $mat.gltf.pbrSpecularGlossiness @ Trade::MaterialAttributeType::String: 
  $mat.twosided @ Trade::MaterialAttributeType::String: 
  $tex.file.LIGHTMAP @ Trade::MaterialAttributeType::String: WaterBottle_occlusion.png
  $tex.file.UNKNOWN @ Trade::MaterialAttributeType::String: !WaterBottle_roughnessMetallic.png
  $tex.file.texCoord.DIFFUSE @ Trade::MaterialAttributeType::String: 
  $tex.file.texCoord.EMISSIVE @ Trade::MaterialAttributeType::String: 
  $tex.file.texCoord.LIGHTMAP @ Trade::MaterialAttributeType::String: 
  $tex.file.texCoord.NORMALS @ Trade::MaterialAttributeType::String: 
  $tex.file.texCoord.SPECULAR @ Trade::MaterialAttributeType::String: 
  $tex.file.texCoord.UNKNOWN @ Trade::MaterialAttributeType::String: 
  $tex.strength.LIGHTMAP @ Trade::MaterialAttributeType::Float: 1
  AlphaBlend @ Trade::MaterialAttributeType::Bool: false
  AlphaMask @ Trade::MaterialAttributeType::Float: 0.5
  BaseColor @ Trade::MaterialAttributeType::Vector4: Vector(1, 1, 1, 1)
  DiffuseColor @ Trade::MaterialAttributeType::Vector4: Vector(1, 1, 1, 1)
  DiffuseTexture @ Trade::MaterialAttributeType::UnsignedInt: 0
  EmissiveColor @ Trade::MaterialAttributeType::Vector3: Vector(1, 1, 1)
  EmissiveTexture @ Trade::MaterialAttributeType::UnsignedInt: 5
  Glossiness @ Trade::MaterialAttributeType::Float: 1
  Metalness @ Trade::MaterialAttributeType::Float: 1
  NormalTexture @ Trade::MaterialAttributeType::UnsignedInt: 3
  NormalTextureScale @ Trade::MaterialAttributeType::Float: 1
  Roughness @ Trade::MaterialAttributeType::Float: 1
  Shininess @ Trade::MaterialAttributeType::Float: 1000
  SpecularColor @ Trade::MaterialAttributeType::Vector4: Vector(1, 1, 1, 1)
  SpecularTexture @ Trade::MaterialAttributeType::UnsignedInt: 6
  Layer 1:
    $tex.file.texCoord.DIFFUSE @ Trade::MaterialAttributeType::String: 
    DiffuseTexture @ Trade::MaterialAttributeType::UnsignedInt: 1