[Type] Fix type conversion in RGBAColor

[alxbilger]

This is to fix the warnings enabled by W4 (#5913) on Sofa.Type

3>RGBAColor.cpp(65,12): Warning C4244 : 'initializing': conversion from 'int' to 'char', possible loss of data
3>RGBAColor.cpp(171,15): Warning C4244 : 'initializing': conversion from 'int' to 'char', possible loss of data
3>RGBAColor.cpp(175,22): Warning C4244 : '=': conversion from 'int' to 'char', possible loss of data
3>RGBAColor.cpp(216,30): Warning C4244 : 'initializing': conversion from 'int' to 'char', possible loss of data
3>RGBAColor.cpp(216,22): Warning C4244 : 'initializing': conversion from 'int' to 'const char', possible loss of data

[with-all-tests]

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[fredroy]

[ci-build][with-all-tests]