change env_rate, env_target and env_level types from unsigned to signed:
fixes issue #50, i.e. the MSVC warnings:
wildmidi_lib.c(1396): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(1406): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(1415): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(1536): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(1546): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(1734): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(1745): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(1879): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(2995): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(3067): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(3336): warning C4146: unary minus operator applied to unsigned type, result still unsigned
wildmidi_lib.c(3408): warning C4146: unary minus operator applied to unsigned type, result still unsigned