Implement TriangleFilter

[wahn]

$ rg -tcpp 'public Filter' ~/git/github/pbrt-v3/src/filters/
...
/home/jan/git/github/pbrt-v3/src/filters/triangle.h
47:class TriangleFilter : public Filter {
...

[wahn]

Commit fbd6ec8 resolves the issue.

> grep '"triangle"' ~/Download/cornell-box/scene.pbrt
PixelFilter "triangle" "float xwidth" [ 1.000000 ] "float ywidth" [ 1.000000 ] 

> imf_diff pbrt.png ~/tmp/pbrt/cornell-box-cpp.png 
pbrt.png /usr/people/jan/tmp/pbrt/cornell-box-cpp.png: no differences.
== "pbrt.png" and "/usr/people/jan/tmp/pbrt/cornell-box-cpp.png" are identical