Cleanup: gcc's -Wwrite-strings warning

[ideasman42]

Quiets -Wwrite-strings warning.
Can cause bugs, though in this case not - just quiet the warnings, so improper use alerts us in future.

toonz/sources/stdfx/igs_line_blur.cpp:117:38: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6635:66: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6639:68: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6643:76: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6647:96: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6656:74: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6676:76: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6684:78: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6694:74: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6698:82: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/stdfx/igs_line_blur.cpp:6702:90: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/CallCircle.cpp:44:39: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/CallCircle.h:136:42: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SDirection.cpp:322:42: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SDirection.cpp:33:43: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SDirection.cpp:378:42: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SDirection.cpp:402:42: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SDirection.cpp:59:43: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SError.h:40:30: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SError.h:55:33: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SError.h:70:32: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/SError.h:85:30: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/STColSelPic.h:55:50: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/STPic.h:130:38: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/STPic.h:543:47: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/STPic.h:564:47: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
toonz/sources/toonzlib/sandor_fxs/STPic.h:606:47: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]

[skitaoka]

Jenkins, test this.

[skitaoka]

Jenkins, test this, again.