`IECoreScene` build failure on architectures with unsigned `char` type

[themissingcow]

Description

Building IECoreScene on an architecture with an unsigned char type results in a warning-as-errors compile failure for:

cortex/src/IECoreScene/Font.cpp

Line 525 in 8669056

if( c < 0 )

(sorry I don't have the log output to hand just now, will re-build when I get a chance)

Steps to Reproduce

1. Build cortex under Alma 9 (gcc 11) on aarch64. (Apple M2)