MAINT-6584: Convert LLImage class hierarchy to standard 'bool'

indra/llimage/llimage.cpp

indra/llimage/llimage.h

indra/llimage/llimagebmp.cpp

indra/llimage/llimagebmp.h

indra/llimage/llimagedxt.cpp

indra/llimage/llimagedxt.h

indra/llimage/llimagej2c.cpp

indra/llimage/llimagej2c.h

indra/llimage/llimagejpeg.cpp

indra/llimage/llimagejpeg.h

indra/llimage/llimagepng.cpp

indra/llimage/llimagepng.h

indra/llimage/llimagetga.cpp

indra/llimage/llimagetga.h

indra/llimagej2coj/llimagej2coj.cpp

indra/llimagej2coj/llimagej2coj.h

indra/llkdu/llimagej2ckdu.cpp

indra/llkdu/llimagej2ckdu.h

indra/llkdu/tests/llimagej2ckdu_test.cpp