Add X-Plane SHADER global attributes

[tngreene]

At the top of the header we're going to want

SHADER_2D
TEXTURE billboards.png
TEXTURE_NORMAL billboards_NML.png

SHADER_3D
TEXTURE trees_3d.png
TEXTURE_NORMAL trees_3d_NML.png

Along with other standards listed here: https://developer.x-plane.com/article/standard-shading-options-for-x-plane-10-scenery/#Availability_of_Extensions (but certainly not all of them)

• [x] Clarify with Petr what he wants
  • TEXTURE
  • TEXTURE_LIT
  • TEXTURE_NORMAL
  • NO_BLEND
  • SPECULAR
  • BUMP_LEVEL
  • NO_SHADOW
  • SHADOW_BLEND
  • NORMAL_METALNESS
  • NORMAL_TRANSLUCENCY (new)
• [x] RNA - where we put it
  1. Exporter looks at 2 string properties (or autodetect from children) in each Forest - the names of the SHADER_2D and SHADER_3D material datablocks
  2. The materials have the properties to set the header directives - with more space in the UI and better ability to re-use between forests
  3. As we figure out what the standard node set up should be we can move properties from the Material Properties tab to the shader setup
• [x] Unit test
• [x] Code

[tngreene]

DIRECTIVE	Source
TEXTURE	Principle BSDF shader's Image Texture Node connected to Base Color
TEXTURE_LIT	?
TEXTURE_NML	Image Texture found by following normal_map?