Fix two UndefVarErrors

[jaakkor2]

Fix for these

using GLTF
o = GLTF.OcclusionTextureInfo()
o.strength = 0.1f0

gives

ERROR: UndefVarError: `strength` not defined
Stacktrace:
 [1] setproperty!(obj::GLTF.OcclusionTextureInfo, sym::Symbol, x::Float32)
   @ GLTF C:\Users\jaakkor2\.julia\packages\GLTF\t9Xw9\src\material.jl:99
 [2] top-level scope
   @ REPL[5]:1

and

using GLTF
p = GLTF.PBRMetallicRoughness()
p.roughnessFactor = 0.1f0

gives

ERROR: UndefVarError: `roughnessFactor` not defined
Stacktrace:
 [1] setproperty!(obj::GLTF.PBRMetallicRoughness, sym::Symbol, x::Float32)
   @ GLTF C:\Users\jaakkor2\.julia\packages\GLTF\t9Xw9\src\material.jl:144
 [2] top-level scope
   @ REPL[3]:1

[jaakkor2]

Ping @Gnimuc

[Gnimuc]

Thanks!