main: use "C" locale for LC_NUMERIC

Some locales use ',' as decimal separator instead of '.', which is taken
into account by sscanf when parsing floating point values.

This can lead to parsing failures for the `colormatrix` and
`forwardmatrix` config options. Setting LC_NUMERIC to "C" solves this
problem.

main.c

@@ -15,6 +15,7 @@
 #include <wordexp.h>
 #include <gtk/gtk.h>
 #include <tiffio.h>
+#include <locale.h>
 #include "config.h"
 #include "ini.h"
 #include "quickdebayer.h"
@@ -1466,6 +1467,8 @@ main(int argc, char *argv[])
 		return ret;
 	}
 
+	setenv("LC_NUMERIC", "C", 1);
+
 	TIFFSetTagExtender(register_custom_tiff_tags);
 
 	gtk_init(&argc, &argv);