Fix outdir quoting issue for tectonic compiler

[thezeroalpha]

For the tectonic compiler: if the path to the file contains spaces, outdir is not quoted properly and hence the build command does not work. This commit fixes that by escaping the path with fnameescape().

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Minimal example of the fixed issue:

1. Create the directory and file:

mkdir ~/'dir with spaces'
touch ~/'dir with spaces'/main.tex

2. Edit main.tex to create a valid document.
3. Set let g:vimtex_compiler_method='tectonic'
4. Try to compile, observe an error. :VimtexCompileOutput shows that 'spaces' in the path is treated as a separate argument, though it should not be.

[lervag]

Thanks! I'll accept this, except I don't want the modern method style. Simply because it breaks from the style used throughout in the VimTeX code - I do generally agree it is a nice style!

[lervag]

Thanks!