Fix const-ness warnings in conversion programs.
Fix compile of png2sm.

I'm assuming that a "PNG heightmap" is greyscale here.
Enable building conversion programs by default.
Remove unused variables.
Fix missing-braces warning.
Remove unnecessary const.
Explicitly init parent class GEL of GESTURE.
Add suggested parentheses and braces.
Remove more set-but-not-used variables.
Fix some additional signed/unsigned errors.
Add extra warnings in debug mode.
Add dependencies to readme and remove 32-bit note.