Fix .pbrt parser

[wahn]

I will collect problems for parsing .pbrt files here and re-open the issue for each parser related problem.

[wahn]

After checking in a new test scene the rendering via the current version (v042) fails:

$ ~/git/github/rs_pbrt/target/release/examples/rs_pbrt -i vw-van.pbrt
pbrt version 0.4.2 [Detected 8 cores]
Copyright (c)2016-2018 Jan Douglas Bert Walter.
Rust code based on C++ code by Matt Pharr, Greg Humphreys, and Wenzel Jakob.
thread 'main' panicked at '  --> 52:9
   |
52 |         "spectrum k" "spds/Al.k.spd"
   |         ^---
   |
   = expected statement, parameter, or last_statement', examples/rs_pbrt.rs:468:80
note: Run with `RUST_BACKTRACE=1` for a backtrace.

[wahn]

Let's try to figure out what has to be changed in the pest based parser:

$ rg spectrum -B 3
vw-van.pbrt
49-MakeNamedMaterial "Metal - Chrome" 
50-        "float roughness" [ 0.01 ] 
51-        "string type" [ "metal" ] 
52:        "spectrum k" "spds/Al.k.spd"
53:        "spectrum eta" "spds/Al.eta.spd"

I think the first problem is that the string/ident "Metal - Chrome" contains spaces:

$ pwd
/home/jan/git/self_hosted/Rust/pbrt/examples
$ grep MakeNamedMaterial pbrt.pest
// MakeNamedMaterial "myplastic" "string type" "plastic" "float roughness" 0.1
make_named_material = { "MakeNamedMaterial" ~ string ~ parameter* }
$ grep "^string =" pbrt.pest
string = { ("\"" ~ ident ~ "\"") | ("\"" ~ filename ~ "\"") }
$ grep "^ident =" pbrt.pest -A 1
ident =  { ('a'..'z' | 'A'..'Z' | "_" | '0'..'9') ~
           ('a'..'z' | 'A'..'Z' | "_" | "-" | ":" | '0'..'9')* }

Let's allow that ...

[wahn]

I think the next problem is this rule:

$ rg "spectrum_param = "
pbrt.pest
55:spectrum_param = { "\"spectrum\"" ~ string }

[wahn]

After commit a2f3c9f we still get a panic:

$ ~/git/github/rs_pbrt/target/release/examples/rs_pbrt -i vw-van.pbrt
pbrt version 0.4.2 [Detected 8 cores]
Copyright (c)2016-2018 Jan Douglas Bert Walter.
Rust code based on C++ code by Matt Pharr, Greg Humphreys, and Wenzel Jakob.
thread 'main' panicked at '  --> 66:17
   |
66 |     ObjectBegin "Null.60"
   |                 ^---
   |
   = expected statement or last_statement', examples/rs_pbrt.rs:468:80
note: Run with `RUST_BACKTRACE=1` for a backtrace.

But those problem(s) are most likely related to the missing handling of keywords like ObjectBegin, ObjectEnd, and ObjectInstance ... We should use another issue for that ...

[wahn]

We still can't handle the three keywords mentioned above correctly, but the parser itself works after commit 3f79674 and I will close the issue for now, until we run into another parser related problem ...