TgaImporter: use better-defined type.

Commit mosra/corrade@9485ef7effd6c8241e03982c53e8fabfe260b525 broke
this, apparently the compiler can't decide which overload to select when
a type has implicit conversion to another type which is then supported
by both Debug and std::ostream operator<<. Working around this for now as
it is the only case, hopefully we won't run into something similar later.