build: fix failed test with old fmt (AcademySoftwareFoundation#4758)

Our fmath_test (a.k.a. unit_fmath) test was failing with an exception
thrown by the fmt library because we used a width specifier when
formatting a TypeDesc.

It was really only an issue when using old fmt < 10.0, so I'm just
solving it trivially by ignoring that specifier for old fmt. It doesn't
seem necessary to make it work correctly with width specifiers for old
versions, when we don't in any way rely on it.

Signed-off-by: Larry Gritz <[email protected]>
Signed-off-by: Scott Wilson <[email protected]>

Author: lgritz

src/include/OpenImageIO/typedesc.h

@@ -616,6 +616,9 @@ struct formatter<OIIO::TypeDesc> {
     {
         // Get the presentation type, if any. Required to be 's'.
         auto it = ctx.begin(), end = ctx.end();
+        // Skip any width specifier. Remember that this is only for old
+        // versions of fmt, so just don't sweat it.
+        while (it != end && isdigit(*it)) ++it;
         if (it != end && (*it == 's')) ++it;
         // Check if reached the end of the range:
         if (it != end && *it != '}')