Load scene render params

src/fart.propane

@@ -1,5 +1,25 @@
 tree;
 tree_prefix P;
+ptype double;
+
+<<header
+/* User terminate codes. */
+#define PARSER_TERMINATE_NUMBER_RANGE 1u
+>>
+
+<<
+#include <charconv>
+
+/* Store the numeric value of the matched text in *out. Returns false if the
+ * value cannot be represented. */
+static bool parse_number(uint8_t const * match_text, size_t match_length, double * out)
+{
+    const char * first = (const char *)match_text;
+    const char * last = first + match_length;
+    std::from_chars_result r = std::from_chars(first, last, *out);
+    return (r.ec == std::errc()) && (r.ptr == last);
+}
+>>
 
 drop /\s+/;
 
@@ -14,8 +34,18 @@ token rcurly /\}/;
 token scene;
 token width;
 
-token uint /\d+/;
-token number /-?\d+(\.\d+)?/;
+token uint /\d+/ <<
+    if (!parse_number(match_text, match_length, &$$))
+    {
+        $terminate(PARSER_TERMINATE_NUMBER_RANGE);
+    }
+>>
+token number /-?\d+(\.\d+)?/ <<
+    if (!parse_number(match_text, match_length, &$$))
+    {
+        $terminate(PARSER_TERMINATE_NUMBER_RANGE);
+    }
+>>
 
 Start -> RenderParams Scene;